• ESO 350-40 Cartwheel

• ESO 510-G13 Warped Edge-On

• M-100 Spiral

• M-101 Pinwheel

• M-104 Sombrero

• M-31 Andromeda

• M-33 Triangulum

• M-51a Whirlpool

• M-64 Blackeye

• M-81 Bode's Galaxy

• M-82 Starburst

• NGC-1300 Barred Spiral

• NGC-1309 Spiral

• NGC-1672 Barred Spiral

• NGC-2207 and IC-2163

• NGC-3314 Overlapping Spirals

• NGC-3370 Silverado

• NGC-3949 Spiral

• NGC-4038/4039 Antennae

• NGC-4414 Spiral

• NGC-4603 Spiral

• NGC-5754/NGC5752 Interacting

• NGC-5866 Spindle

• NGC-7674 Interacting

• UGC-9618 Interacting

Back to Top
Messier 100 (also known as NGC 4321) is a spiral galaxy about 52.5 million light-years away in the constellation Coma Berenices. It was discovered by Pierre Méchain in 1781. It is one of the brightest galaxies in the Virgo cluster. Five supernovae have been identified in M100: SN 1901B, SN 1914A, SN 1959E, SN 1979C and SN 2006X. M100 also has a satellite galaxy named NGC 4323.

like the Milky Way, M-100 is tilted nearly face-on as seen from earth. It is among the first spirals that have been discovered, and listed by Lord Rosse as one of 14 "spiral nebulae" discovered to 1850. The galaxy has two prominent arms of bright blue stars and several fainter arms. The blue stars in the arms are young hot and massive stars which formed recently from density perturbations caused by interactions with neighboring galaxies which are lying just outside our image. Despite its nearly perfect symmetric outline, this galaxy appears slightly asymmetric, as on the southern (lower) side of the nucleus more (or brighter) young stars have formed.

Back to Top
The Pinwheel Galaxy (also known as Messier 101 or NGC 5457) is a face-on spiral galaxy about 27 million light-years away in the constellation Ursa Major.

It was discovered by Pierre Méchain on March 27, 1781, and he subsequently communicated his discovery to Charles Messier who verified its position and added it to the Messier Catalogue as one of the final entries.

On February 28, 2006, NASA and the ESA released a very detailed image of Pinwheel Galaxy, which was the largest and most detailed image of a galaxy by Hubble Space Telescope at the time. The image was composed from 51 individual exposures, plus some extra ground-based photos.

M101 is a relatively large galaxy compared to the Milky Way. With a diameter of 170,000 light-years it is nearly twice the size of the Milky Way. Less is known about the mass of M101. A frequently cited number is an equivalent mass of about 16 billion solar masses. That value is almost certainly too low, and probably stems from M101's low surface brightness. New insights in its HII regions and rotational velocities have put the number between 100 billion and 1 trillion suns.

Another remarkable property of this galaxy are its huge and extremely bright HII regions, of which a total of about 3000 can be seen on photographs. HII regions are places in galaxies that contain enormous clouds of high density hydrogen gas contracting under its own gravitational force. Eventually, when the localized hydrogen contracts enough for fusion processes to begin, stars are born. Consequently, HII regions are places that often contain large numbers of extremely bright and hot young stars giving them their characteristic blue color.

On photographs M101 can be seen to be asymmetrical on one side. It is thought that in the recent past (speaking in galactic terms) M101 underwent a near collision with another galaxy and the associated gravitational tidal forces caused the asymmetry. In addition, this encounter also amplifies the density waves in the spiral arms of M101. The amplification of these waves also leads to the compression of the interstellar hydrogen gas, which then triggers strong star formation activity.

M101 has five prominent companion galaxies: NGC 5204, NGC 5474, NGC 5477, NGC 5585, and Holmberg IV. As stated above, the gravitational interaction between M101 and its satellites may have triggered the formation of the grand design pattern in M101. M101 has also problably distorted the companion galaxy NGC 5474. M101 and its companion galaxies comprise most or possibly all of the M101 Group.

Back to Top
The Sombrero Galaxy (also known as M104 or NGC 4594) is an unbarred spiral galaxy in the constellation Virgo. It has a bright nucleus, an unusually large central bulge, and a prominent dust lane in its inclined disk. The dark dust lane and the bulge give this galaxy the appearance of a sombrero. The galaxy has an apparent magnitude of 9.0, making it a galaxy that can easily be seen with amateur telescopes. The large bulge, the central supermassive black hole, and the dust lane all attract the attention of professional astronomers.

As noted above, this galaxy's most striking feature is the dust lane that crosses in front of the bulge of the galaxy. This dust lane is actually a symmetric ring that encloses the bulge of the galaxy.^[5] Most of the cold atomic hydrogen gas and the dust lies within this ring. The ring might also contain most of the Sombrero Galaxy's cold molecular gas, although this is an inference based on observations with low resolution and weak detections. Additional observations are needed to confirm that the Sombrero galaxy's molecular gas is constrained to the ring. Based on infrared spectroscopy, the dust ring is the primary site of star formation within this galaxy.

The nucleus of the Sombrero galaxy is classified as a low ionization nuclear emission region (LINER). These are nuclear regions where ionized gas is present, but the ions are only weakly ionized (i.e. the atoms are missing relatively few electrons). The source of energy for ionizing the gas in LINERs has been debated extensively. Some LINER nuclei may be powered by hot, young stars found in star formation regions, whereas other LINER nuclei may be powered by active galactic nuclei (highly energetic regions that contain supermassive black holes). Infrared spectroscopy observations have demonstrated that the nucleus of the Sombrero Galaxy is probably devoid of any significant star formation activity. However, a supermassive black hole has been identified in the nucleus (as discussed in the subsection below), so this active galactic nucleus is probably the energy source that weakly ionizes the gas in the Sombrero Galaxy.

Central supermassive black hole

In the 1990s, a research group led by John Kormendy demonstrated that a supermassive black hole is present within the Sombrero Galaxy. Using spectroscopy data from both the CFHT and the Hubble Space Telescope, the group showed that the speed of rotation of the stars within the center of the galaxy could not be maintained unless a mass 1 billion times the mass of the Sun, or 10⁹M_☉, is present in the center. This is among the most massive black holes measured in any nearby galaxies.

Synchrotron emission

At radio and X-ray wavelengths, the nucleus is a strong source of synchrotron emission. Synchrotron emission is produced when high velocity electrons oscillate as they pass through regions with strong magnetic fields. This emission is actually quite common for active galactic nuclei. Although radio synchrotron emission may vary over time for some active galactic nuclei, the luminosity of the radio emission from the Sombrero Galaxy only varies 10-20%.

Unidentified submillimeter emission

In 2006, two groups published measurements of the submillimeter radiation from the nucleus of the Sombrero Galaxy at a wavelength of 850 micrometres. This submillimeter emission was found not to originate from the thermal emission from dust (which is commonly seen at infrared and submillimeter wavelengths), synchrotron emission (which is commonly seen at radio wavelengths), bremsstrahlung emission from hot gas (which is uncommonly seen at millimeter wavelengths), or molecular gas (which commonly produces submillimeter spectral lines). The source of the submillimeter emission remains unidentified.

Globular clusters

The Sombrero Galaxy has a relatively large number of globular clusters. Observational studies of globular clusters in the Sombrero Galaxy have produced estimates of the population in the range of 1200 to 2000. The ratio of the number of globular clusters to the total luminosity of the galaxy is high compared to the Milky Way and similar galaxies with small bulges, but the ratio is comparable to other galaxies with large bulges. These results have been repeatedly used to demonstrate that the number of globular clusters in galaxies is thought to be related to the size of the galaxies' bulges. The surface density of the globular clusters generally follows the light profile of the bulge except for near the center of the galaxy.

Distance

At least two methods have been used to measure the distance to the Sombrero Galaxy.

The first method relies on comparing the measured fluxes from planetary nebulae in the Sombrero Galaxy to the known luminosities of planetary nebulae in the Milky Way. This method gave the distance to the Sombrero Galaxy as 29.0 ± 2.0 Mly (8.9 ± 0.6 Mpc).

The other method used is the surface brightness fluctuations method. This method uses the grainy appearance of the galaxy's bulge to estimate the distance to it. Nearby galaxy bulges will appear very grainy, while more distant bulges will appear smooth. Early measurements using this technique gave distances of 30.6 ± 1.3 Mly (9.4 ± 0.4 Mpc). Later, after some refinement of the technique, a distance of 32 ± 3 Mly (9.8 ± 0.8 Mpc) was measured. This was even further refined in 2003 to be 29.6 ± 2.5 Mly (9.1 ± 0.8 Mpc).

The average distance measured through these two techniques is 29.3 Mly (9.0 Mpc) with an uncertainty of 1.6 Mly (0.5 Mpc).

Nearby galaxies and galaxy group information

The Sombrero Galaxy lies within a complex, filament-like cloud of galaxies that extends to the south of the Virgo Cluster. However, it is unclear as to whether the Sombrero Galaxy is part of a formal galaxy group. Hierarchical methods for identifying groups, which determine group membership by considering whether individual galaxies belong to a larger aggregate of galaxies, typically produce results showing that the Sombrero Galaxy is part of a group that includes NGC 4487, NGC 4504, NGC 4802, UGCA 289, and possibly a few other galaxies. However, results that rely on the percolation method (i.e. the "friends-of-friends" method), which links individual galaxies together to determine group membership, indicate that either the Sombrero Galaxy is not in a group or that it may only be part of a galaxy pair with UGCA 287.

Back to Top
The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda Nebula in older texts) is a spiral galaxy approximately 2.5 million light-years away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way. As it is visible as a faint smudge on a moonless night, it is one of the farthest objects visible to the naked eye, and can be seen with binoculars even in urban areas.

Andromeda is the largest galaxy of the Local Group, which consists of the Andromeda Galaxy, the Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, it may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping. However, recent observations by the Spitzer Space Telescope revealed that M31 contains one trillion (10¹²) stars, greatly exceeding the number of stars in our own galaxy. 2006 estimates put the mass of the Milky Way to be ~80% of the mass of Andromeda, which is estimated to be 7.1×10¹¹ solar masses.

At an apparent magnitude of 4.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects, making it easily visible to the naked eye even when viewed from areas with moderate light pollution. It appears quite small without a telescope because only the central part is bright enough to be visible, but the full angular diameter of the galaxy is seven times that of the full moon.

The Andromeda Galaxy is approaching the Sun at about 300 kilometers per second (186 miles/s.), so it is one of the few blue shifted galaxies. Given the motion of the Solar System inside the Milky Way, one finds that the Andromeda Galaxy and the Milky Way are approaching one another at a speed of 100 to 140 kilometers per second (62–87 miles/s.; 223,200–313,200mph). The impact is predicted to occur in about 2.5 billion years. In that case the two galaxies will likely merge to form a giant elliptical galaxy. However, Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two, which creates uncertainty about the details of when the collision will take place and how it will proceed. Such events are frequent among the galaxies in galaxy groups.

The measured distance to the Andromeda Galaxy was doubled in 1953 when it was discovered that there is another, dimmer type of Cepheid. In the 1990s, Hipparcos satellite measurements were used to calibrate the Cepheid distances. The corrected value gives the Andromeda Galaxy a distance of 2.9 million light-years. Unfortunately, all Cepheids lie farther than Hipparcos could measure accurately, and it became clear that Hipparcos-calibrated values for Cepheids were not reliable.

Back to Top
The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a spiral galaxy approximately 3 million light-years away in the constellation Triangulum. The galaxy is also sometimes informally referred to as the Pinwheel Galaxy by some amateur astronomy references and in some public outreach websites.^[7] However, the SIMBAD Astronomical Database, a professional astronomy database that contains formal designations for astronomical objects, indicates that the name "Pinwheel Galaxy" is used to refer to Messier 101, and several other amateur astronomy resources and other public outreach websites also identify Messier 101 by that name. It is the third largest galaxy in the Local Group, a group of galaxies that also contains the Milky Way Galaxy and the Andromeda Galaxy, and it may be a gravitationally bound companion of the Andromeda Galaxy. The Pisces Dwarf (LGS 3), one of the small Local Group member galaxies, is possibly a satellite of Triangulum.

The Triangulum Galaxy can be seen with the naked eye under exceptionally good conditions. While the fainter and more distant galaxy Messier 81 has also been seen with the naked eye by very experienced observers, M33 is often considered the most distant object that can be seen without aid.^[6] However, some amateur astronomers may confuse the object with the nearby NGC 752, an open cluster that is brighter than the Triangulum Galaxy. No known pre-telescopic observer notes it, which is not surprising: given its indistinctness, it is not likely to be noticed as an object unless one already knows of its existence.

The Triangulum Galaxy was probably discovered by Giovanni Batista Hodierna before 1654, who may have grouped it together with open cluster NGC 752. It was independently discovered by Charles Messier in 1764, who catalogued it as M33 on August 25. M33 was also catalogued independently by William Herschel on September 11, 1784 number H V.17. It was among the first "spiral nebulae" identified as such by Lord Rosse.

Herschel also cataloged The Triangulum Galaxy's brightest and largest H II region (diffuse emission nebula containing ionized hydrogen) as H III.150 separately from the galaxy itself, which eventually obtained NGC number 604. As seen from Earth NGC 604 is located northeast of the galaxy's central core, and is one of the largest H II regions known with a diameter of nearly 1500 light-years and a spectrum similar to the Orion Nebula. Herschel also noted 3 other smaller H II regions (NGC 588, 592 and 595).

In 2005, using observations of two water masers on opposite sides of Triangulum via the VLBA, researchers were, for the first time, able to estimate the angular rotation and proper motion of Triangulum. A velocity of 190 ± 60 km/s relative to the Milky Way is computed which means Triangulum is moving towards Andromeda.

In 2007, a black hole about 15.7 times the mass of the Sun was detected in the galaxy using data from the Chandra X-ray Observatory. The black hole, named M33 X-7, orbits a companion star which it eclipses every 3.5 days.

The galaxy has an H II nucleus.

At least three techniques have been used to measure distances to M 33. Using the Cepheid variable method, an estimate of 2.77 ± 0.13 Mly (850 ± 40 kpc) was achieved in 2004.

Also 2004, the Tip of the Red Giant Branch (TRGB) method was used to derive a distance estimate of 2.59 ± 0.08 Mly (794 ± 23 kpc).

In 2006, a group of astronomers announced the discovery of an eclipsing binary star in the Triangulum Galaxy. By studying the eclipses of the stars, the astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars they were able to measure the absolute magnitude of the stars. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of 3.1 ± 0.2 Mly (940 ± 70 kpc).

Averaged together, all these distance measurements give a combined distance estimate of 2.81 ± 0.09 Mly (861 ± 28 kpc).

Back to Top
The Whirlpool Galaxy (also known as Messier 51a, M51a, or NGC 5194) is an interacting grand-design spiral galaxy located at a distance of approximately 23 million light-years in the constellation Canes Venatici. It is one of the most famous spiral galaxies in the sky. The galaxy and its companion (NGC 5195) are easily observed by amateur astronomers, and the two galaxies may even be seen with binoculars. The Whirlpool Galaxy is also a popular target for professional astronomers, who study it to further understand galaxy structure (particularly structure associated with the spiral arms) and galaxy interactions.

The Whirlpool Galaxy was discovered by Charles Messier on October 13, 1773. Its companion galaxy, NGC 5195, was discovered in 1781 by Pierre Méchain. It was however not until 1845 that the Whirlpool became the first galaxy to be recognized as a spiral. This was achieved by Lord Rosse employing a 72-inch reflecting telescope which he constructed at Birr Castle, Ireland. In 2005 a supernova (SN 2005cs) was observed in the Whirlpool Galaxy, peaking at apparent magnitude 14. Sometimes M51 is used to refer to the pair of galaxies, in which case the individual galaxies may be referred to as M51A (NGC 5194) and M51B (NGC 5195).

Properties

With the recent SN 2005cs derived estimate of 23 Mly distance, and an angular diameter of roughly 11.2′, it can be inferred that M51's bright circular disk has a radius of about ~38,000 light-years. Its mass is estimated to be 160 billion solar masses. Compared to 100 kly diameter of the Milky Way, M51 has about half its size and mass.

A black hole, surrounded by a ring of dust, is thought to exist at the heart of the spiral. The dust ring stands almost perpendicular to the relatively flat spiral galaxy. A secondary ring crosses the primary ring on a different axis, a phenomenon that is contrary to expectations. A pair of ionization cones extend from the axis of the main dust ring.

Visual appearance

Located within the constellation Canes Venatici, M51 is easy to find by following the easternmost star of the Big Dipper, Eta Ursae Majoris, and going 3.5° southeast. Its declination is +47°, so it is circumpolar for observers located above 43°N latitude and reaches high altitudes throughout the northern hemisphere making it an accessible object, especially from the very early hours in winter through the end of spring season, after which summer solstice somewhat hinders observations.

M51 is visible through binoculars on a dark night, and can be resolved in detail with modern amateur telescopes. When seen through a 10 cm telescope the basic outlines of M51 and its companion are visible. Under dark skies, and with a moderate eyepiece through a 15 cm telescope, M51's intrinsic spiral structure can be detected. With larger (>30 cm) instruments, the various spiral bands are apparent and several HII regions appear to be visible, and M51 can be seen to be attached to M51B.

As is usual for galaxies, the true extent of its size can only be gathered from inspecting deep photographs, and very long exposures reveal a large nebula extending beyond the visible circular appearance.

In January 2005 the Hubble Heritage Team constructed a 11477x7965 pixel composite image (shown in the info box above) of M51 using Hubble's ACS instrument, revealing the galaxy and its companion in unprecedented detail.

Spiral structure

The very pronounced spiral structure of the Whirlpool Galaxy is believed to be the result of the close interaction between it and its companion galaxy NGC 5195.

Star formation

Induced spiral structure in the larger galaxy isn't the only effect of the interaction. Significant compression of hydrogen gas occurs that leads to the development of starbirth regions. In pictures of M51 these show up as the bright blue 'knots' throughout the spiral arms.

Generally speaking, hydrogen gas is the most common component of the interstellar medium (the vast space between stars and planetary systems in galaxies). It exists primarily in its atomic and molecular form, and forms huge clouds throughout the entire galaxy. When large sources of gravitational pull pass nearby, such as other galaxies, gravitational interactions produce compression (density) waves that sweep through these hydrogen clouds. This causes some regions of the previously diffuse gas to compress into tight pockets of opaque and dense gas, these are dust lanes one so often sees in the spiral arms. In regions where the concentration and density of gas reaches a critical value, further collapse under its own gravitational pull occurs, and stars are born at the center of the collapse, where the gas is compressed so strongly that fusion initiates.

When this happens, these new-born stars gobble up huge amounts of gas causing them to expand, shine even hotter, and finally sweep away the surrounding layers of dust and gas by increasing efflux of the stellar wind. The gigantic proportions of the clouds out of which they are born means stars seldom, if ever, are created in isolation. Thus regions of several hot young stars emit sufficient light energy that they can be seen in the high resolution pictures of M51 across millions of lightyears distance.

For an example of such a formation in our own galaxy, see M16, the Eagle Nebula.

Companion

Decades ago, it was not known with certainty whether the companion galaxy NGC 5195 was a true companion, or another galaxy passing at a distance. The advent of radio astronomy and subsequent radio images of M51 unequivocally demonstrated the reality of the interaction.

Recent simulations bear out that M51's spiral structure was caused by NGC 5195 passing through the main disk of M51 about 500 to 600 million years ago. In this model, NGC 5195 came from behind M51 through the disk towards the observer and made another disk crossing as recently as 50 to 100 Myrs ago until it is where we observe it to be now, slightly behind M51.

Back to Top
The Black Eye Galaxy (also called Sleeping Beauty Galaxy; designated Messier 64, M64, or NGC 4826) was discovered by Edward Pigott in March 1779, and independently by Johann Elert Bode in April of the same year, as well as by Charles Messier in 1780. It has a spectacular dark band of absorbing dust in front of the galaxy's bright nucleus, giving rise to its nicknames of the "Black Eye" or "Evil Eye" galaxy. M64 is well known among amateur astronomers because of its appearance in small telescopes. It is a spiral galaxy in the Coma Berenices constellation.

At first glance, M64 seems to be a fairly normal spiral galaxy. As in the majority of galaxies, all of the stars in M64 are orbiting in the same direction, clockwise as seen in the Hubble image.

However, recent detailed studies have led to the remarkable discovery that the interstellar gas in the outer regions of M64 rotates in the opposite direction from the gas and stars in the inner regions. The inner region has a radius of only approximately 3,000 light-years, while the outer section extends another 40,000 light-years. This pattern is believed to trigger the creation of many new stars around the boundary separating the two regions.

A collision of two galaxies has left a merged star system with an unusual appearance as well as bizarre internal motions. Astronomers believe that the oppositely rotating gas arose when M64 absorbed a satellite galaxy that collided with it, perhaps more than one billion years ago. Active formation of new stars is occurring in the shear region where the oppositely rotating gases collide, are compressed, and contract.

Particularly noticeable in the image are hot, blue young stars that have just formed, along with pink clouds of glowing hydrogen gas that fluoresce when exposed to ultraviolet light from newly formed stars. It is approximately 17 million light years from earth

The small galaxy that impinged on its neighbour has now been almost completely destroyed, its stars either merged with the main galaxy or scattered into space, but signs of the collision persist in the backward motion of gas at the outer edge of M64.

Back to Top
Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a spiral galaxy about 12 million light-years away in the constellation Ursa Major. M81 is one of the most striking examples of a grand design spiral galaxy, with near perfect arms spiraling into the very center. Because of its proximity to Earth, its large size, and its active galactic nucleus (which may harbor a supermassive black hole) Messier 81 is a popular galaxy to study in professional astronomy research. The galaxy's large size and relatively low apparent magnitude (lower magnitude implies higher brightness) also make it a popular target for amateur astronomy observations.

Messier 81 was first discovered by Johann Elert Bode in 1774. Consequently, the galaxy is sometimes referred to as "Bode's Galaxy". In 1779, Pierre Méchain and Charles Messier reidentified Bode's object, which was subsequently listed in the Messier Catalogue.

Dust emission

Most of the emission at infrared wavelengths originates from interstellar dust. This interstellar dust is found primarily within the galaxy's spiral arms, and it has been shown to be associated with star formation regions. The general explanation is that the hot, short-lived blue stars that are found within star formation regions are very effective at heating the dust and hence enhancing the infrared dust emission from these regions.

Supernova

Only one supernova has been detected in Messier 81. The supernova, named SN 1993J, was discovered on 28 March 1993 by F. Garcia in Spain. At the time, it was the second brightest supernova observed in the twentieth century. The spectral characteristics of the supernova changed over time. Initially, it looked more like a type II supernova (a supernovae formed by the explosion of a giant star) with strong hydrogen spectral line emission, but later the hydrogen lines faded and strong helium spectral lines appeared, making the supernova look more like a type Ib. Moreover, the variations in SN 1993J's luminosity over time were not like the variations observed in other type II supernovae but did resemble the variations observed in type Ib supernovae. Hence, the supernova has been classified as a "type IIb", a transitory class between type II and type Ib. The scientific results from this supernova suggested that type Ib and Ic supernovae were actually formed through the explosions of giant stars through processes similar to what takes place in type II supernovae. The supernova was also used to estimate a distance of 8.5 ± 1.3 Mly (2.6 ± 0.4 Mpc) to Messier 81.

Nearby galaxies and galaxy group information

Messier 81 is the largest galaxy in the M81 Group, a group of 34 galaxies located in the constellation Ursa Major. The distance from the Earth to the group is approximately 11.7 Mly (3.6 Mpc), making this one of the closest groups to the Local Group, which contains the Milky Way.

M81 is gravitationally interacting with Messier 82 and NGC 3077. The interactions have stripped some hydrogen gas away from all three galaxies, leading to the formation of filamentary gas structures in the group. Moreover, the interactions have also caused some interstellar gas to fall into the centers of Messier 82 and NGC 3077, which has led to strong starburst activity (or the formation of many stars) within the centers of these two galaxies.

Back to Top
Messier 82 (also known as NGC 3034 or the Cigar Galaxy) is the prototype nearby starburst galaxy about 12 million light-years away in the constellation Ursa Major. The starburst galaxy is five times as bright as the whole Milky Way and one hundred times as bright as our galaxy's center.

In 2005, the Hubble revealed 197 young massive clusters in the starburst core. The average mass of these clusters is around 2×10⁵ M_⊙, hence the starburst core is a very energetic and high-density environment. Throughout the galaxy's center, young stars are being born 10 times faster than they are inside our entire Milky Way Galaxy.

Forming a striking pair in small telescopes with nearby spiral M81, M82 is being physically affected by its larger neighbor. Tidal forces caused by gravity have deformed this galaxy, a process that started about 100 million years ago. This interaction has caused star formation to increase 10 fold compared to "normal" galaxies.

Recently, M82 has undergone at least one tidal encounter with M81 resulting in a large amount of gas being funneled into the galaxy's core over the last 200 Myr. The most recent such encounter is thought to have happened around 2–5×10⁸ years ago and resulted in a concentrated starburst together with a corresponding marked peak in the cluster age distribution. This starburst ran for up to ~50 Myr at a rate of ~10 M_⊙ per year. Two subsequent startbursts followed, the last (~4–6 Myr ago) of which may have formed the core clusters, both super star clusters (SSCs) and their lighter counterparts.

Ignoring any difference in their respective distances from us, the centers of M81 and M82 are visually separated by about 130,000 light-years. The actual separation is 300⁺³⁰⁰₋₂₀₀ kly.

Starburst region

In the core of M82, the active starburst region spans a diameter of 500 pc. In optical, there are four high surface brightness regions or clumps (designated A, C, D, and E). These clumps correspond to known sources at X-ray, infrared, and radio frequencies. Consequently, they are thought to be the least obscured starburst clusters from our vantage point. M82's unique bipolar outflow (or 'superwind') appears to be concentrated on clumps A and C and fueled by the energy injected by supernova that occur about once every ten years.

The Chandra X-ray Observatory detected fluctuating X-ray emissions from a location approximately 600 light-years away from the center of M82. Astronomers have postulated that this fluctuating emission comes from the first known intermediate-mass black hole, of roughly 200 to 5000 solar masses. M82, like most galaxies, hosts a supermassive black hole at its center with a mass of approximately 3 x 10⁷ solar masses as measured from stellar dynamics.

Structure

M82 was previously believed to be an irregular galaxy. However, in 2005, two symmetric spiral arms were discovered in the near-infrared (NIR) images of M82. The arms were detected by subtracting an axisymmetric exponential disk from the NIR images. These arms emanate from the ends of the NIR bar and can be followed for the length of 3 disc scales. Even though the arms were detected in the NIR images, they are bluer than the disk. Assuming that the northern part of M82 is nearer to us, which most literature assumes, the observed sense of rotation implies trailing arms. Due to M82's high disk surface brightness, nearly edge-on orientation (~80°) with respect to us, and the presence of a complex network of dusty filaments in optical images, the arms were not previously detected.

Back to Top
NGC 1300 is a barred spiral galaxy about 69 million light-years away in the constellation Eridanus and is part of the Eridanus Cluster. It was discovered by John Frederick William Herschel in 1835.

In 2005 the Hubble Space Telescope took a look at NGC 1300. Its resolution, a myriad of fine details, some of which have never before been seen, is seen throughout the galaxy's arms, disk, bulge, and nucleus. Blue and red supergiant stars, star clusters, and star-forming regions are well resolved across the spiral arms, and dust lanes trace out fine structures in the disk and bar. Numerous more distant galaxies are visible in the background, and are seen even through the densest regions of NGC 1300.

In the core of the larger spiral structure of NGC 1300, the nucleus shows its own extraordinary and distinct "grand-design" spiral structure that is about 3,300 light-years long. Only galaxies with large-scale bars appear to have these grand-design inner disks — a spiral within a spiral. Models suggest that the gas in a bar can be funneled inwards, and then spiral into the center through the grand-design disk, where it can potentially fuel a central black hole. NGC 1300 is not known to have an active nucleus, however, indicating either that there is no black hole, or that it is not accreting matter. The galaxy is roughly 150,000 light-years in diameter.

Back to Top
NGC 1672 is a barred spiral galaxy located in the constellation Dorado. It was originally thought to be a member of the Dorado Group, however, this membership was later rejected. NGC 1672 has a large bar which is estimated to measure around 20 kpc. It has very strong radio emissions emanating from its nucleus, bar, and the inner portion of the spiral arm region. The nucleus is Seyfert type 2 and is engulfed by a starburst region. The strongest polarized emissions come from the northeastern region which is upstream from its dust lanes. Magnetic field lines are at large angles with respect to the bar and turn smoothly to the center.

The center of the galaxy contains a high surface brightness bar, and four filament-like spiral arms extend outward from the ends of this bar. The spiral arms are asymmetric; one of the arms in the northeast part of the disk is significantly brighter than its counterpart on the other side. The spiral arms also contain numerous star formation regions, some of which may be as large as 4′′.

The classification of the nucleus of NGC 1672 is uncertain. Most galaxies may be classified by their spectra as having one of three different types of nuclei:

• A nuclear HII region, a region which has a spectrum similar to that of star formation regions in the Milky Way and which therefore is associated with nuclear star formation activity.
• A Seyfert nucleus, a type of active galactic nucleus (AGN) that may contain a supermassive black hole.
• A low ionization nuclear emission-line region, a type of nuclear region with spectral line emission from weakly ionized gas that could contain either a star formation region or a supermassive black hole.

NGC 1672, however, is one of several nearby galaxies that does not fit into this classification scheme, as its spectrum appears intermediary between these three classes of objects. It may in fact contain both nuclear star formation regions and an AGN. In some wave bands (such as in ultraviolet light), the star formation regions are the primary source of emission.

Back to Top
NGC 2207 and IC 2163 are a pair of spiral galaxies about 144 million light-years away in the constellation Canis Major. Both galaxies were discovered by John Herschel in 1835. So far three supernovae have been observed in NGC 2207 (SN 1975A, SN 1999ec and SN 2003H). NGC 2207 is in the process of tidal stripping IC 2163.

In November 1999, the Hubble Space Telescope took a look at these galaxies.

NGC 2207 is in the process of colliding and merging with IC 2163. But unlike the Antennae or the Mice Galaxies; they're still two separate spiral galaxies. They are only in the first step of colliding and merging. Soon they will collide looking a bit more like the Mice. In about a billion years time they will merge and become an elliptical galaxy

Back to Top
NGC 3370 (also known as UGC 5887, nicknamed Silverado Galaxy) is a spiral galaxy about 98 million light-years away in the constellation Leo. It is similar in size to our own Milky Way. NGC 3370 exhibits intricate spiral arm structure that is sprinkled with hot areas where new stars are forming. NGC 3370's center has well defined dust lanes and an unusually ill-defined nucleus. This galaxy contains a combination of young stars in the bluer regions and older stars in the yellowish core. The mass of NGC 3370 is estimated to be about the same as our own Milky Way at around 10¹¹ solar masses.

NGC 3370 was observed (probably discovered) by William Herschel who gave it the designation II 81 and John Herschel who gave it the designation 750. The object has a surface brightness of 13 and a position angle of 140°. William Herschel cataloged II 80 to NGC 3348 before and II 82 to NGC 3455 after NGC 3370.

On November 14, 1994, S. Van Dyk and the Leuschner Observatory Supernova Search took an image that led to their discovery of a bright supernova in NGC 3370.^[2] This was well before the maximal light of the supernova which has been estimated to have occurred in the range November 30 to December 1. This stellar outburst briefly outshone all of the tens of billions of other stars in its galaxy. Although supernova are common, with one exploding every few seconds somewhere in the universe, this one was special. Designated SN 1994ae, this supernova was one of the nearest and best observed supernova since the advent of modern digital detectors. The supernova was also a member of a special subclass of supernovae, the type Ia, the best tool astronomers have to chart the growth rate of the expanding universe. The supernova was located at 10^h 44^m 21.52^s +17° 32′ 20.7′′.

Back to Top
NGC 3949 is an unbarred spiral galaxy in the constellation Ursa Major. It is believed to be approximately 50 million light-years away from the earth.

The type II supernova SN 2000db is the only supernova that has been observed within NGC 3949.

NGC 3949 is a member of the M109 Group, a group of galaxies located in the constellation Ursa Major that may contain over 50 galaxies. The brightest galaxy in the group is the spiral galaxy M109.

Back to Top
The Antennae Galaxies (also known as NGC 4038/NGC 4039) are a pair of interacting galaxies in the constellation Corvus. They were both discovered by Friedrich Wilhelm Herschel in 1785.

The Antennae are undergoing a galactic collision. Located in the NGC 4038 group with five other galaxies, these two galaxies are known as the 'Antennae' because the two long tails of stars, gas and dust thrown out of the galaxies as a result of the collision resemble the antennae of an insect. The nuclei of the two galaxies are joining to become one giant galaxy. Most galaxies probably undergo at least one significant collision in their lifetimes. This is likely the future of our Milky Way when it collides with the Andromeda Galaxy. Two supernovae have been discovered in the galaxy: SN 2004GT and SN 2007sr.

A recent study finds that these interacting galaxies are closer to the Milky Way than previously thought—at 45 million light-years instead of 65 million light-years.

About 1.2 billion years ago, the Antennae were two separate galaxies. NGC 4038 was a spiral galaxy and NGC 4039 was a barred spiral galaxy. Before the galaxies collided, NGC 4039 was larger than NGC 4038. 900 million years ago, the Antennae began to approach one another, looking similar to NGC 2207 and IC 2163. 600 million years ago, the Antennae passed through each other, looking like the Mice Galaxies. 300 million years ago, the Antennae's stars began to be released from both galaxies. Today the two streamers of ejected stars extend far beyond the original galaxies, making the antennae shape.

Within 400 million years, the Antennae's nuclei will collide and become a single core with stars, gas, and dust around it. Observations and simulations of colliding galaxies suggest that the Antennae Galaxies will eventually form an elliptical galaxy.

Back to Top
NGC 4603, a galaxy with majestic spiral arms and intricate dust lanes, is 108 million light-years away. Its distance has been accurately measured by astronomers using one of the fundamental yardsticks of the extragalactic distance scale - pulsating variable stars known as Cepheids. Though intrinsically very bright, Cepheids are faint and difficult to find at such large distances (the bright "spiky" stars seen above are foreground objects). Thanks to the Hubble Space Telescope's sharp vision, more than 36 beckoning Cepheids have been identified in NGC 4603, now the most distant galaxy in which these stars have been located.

Back to Top
NGC 5754/5752. This beautiful pair of interacting galaxies consists of NGC 5754, the large spiral on the right, and NGC 5752, the smaller companion in the bottom left corner of the image. NGC 5754's internal structure has hardly been disturbed by the interaction. The outer structure does exhibit tidal features, as does the symmetry of the inner spiral pattern and the kinked arms just beyond its inner ring. In contrast, NGC 5752 has undergone a starburst episode, with a rich population of massive and luminous star clusters clumping around the core and intertwined with intricate dust lanes. The contrasting reactions of the two galaxies to their interaction are due to their differing masses and sizes. NGC 5754 is located in the constellation Bootes, the Herdsman, some 200 million light-years away.

Back to Top
NGC 5866 (also called the Spindle Galaxy) is a relatively bright lenticular galaxy in the constellation Draco. Some astronomers believe that the NGC 5866 may be a candidate for Messier 102 NGC 5866 was probably discovered by Pierre Méchain or Charles Messier in 1781, and independently found by William Herschel in 1788.

One of the most outstanding features of NGC 5866 is the extended dust disk, which is seen exactly edge-on. This dust disk is highly unusual for a lenticular galaxy. The dust in most lenticular galaxies is generally found only near the nucleus and generally follows the light profile of the galaxies' bulges. This dust disk may contain a ring-like structure, although the shape of this structure is difficult to determine given the edge-on orientation of the galaxy. It is also possible that the galaxy is a spiral galaxy that was misclassified as a lenticular galaxy because of its edge-on orientation, in which case the dust disk would not be too unusual.

NGC 5866 is one of the brightest galaxies in the NGC 5866 Group, a small galaxy group that also includes the spiral galaxies NGC 5879 and NGC 5907. This group may actually be a subclump at the northwest end of a large, elongated structure that comprises the M51 Group and the M101 Group, although most sources distinguish the three groups as separate entities.

NGC 7674 has a powerful active nucleus of the kind known as a type 2 Seyfert that is perhaps fed by gas drawn into the centre through the interactions with the companions.

NGC 7674 falls into the family of luminous infrared galaxies and is featured in Arp's Atlas of Peculiar Galaxies as number 182. It is located in the constellation of Pegasus, the Winged Horse, about 400 million light-years away from Earth.

This image is part of a large collection of 59 images of merging galaxies taken by the Hubble Space Telescope and released on the occasion of its 18th anniversary on 24 April 2008.