M74, a spiral galaxy in Pisces.
An Sc galaxy containing 100 billion stars, approximately 30 million light years away. Somewhat smaller than our own galaxy, but otherwise very similar in structure. (Gemini Observatory, GMOS Team, apod030524)
is an Sc galaxy in Sculptor
, seen nearly from its side. It lies about 10 million light years away, in the Sculptor Group of galaxies, the nearest group to our own Local Group of galaxies. (Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT, apod030525; © CFHT; used by permission)
Below, another image of NGC253, showing the glowing mass of stars in its extended disk, intermixed with complex dust lanes. (T.A. Rector/University of Alaska Anchorage, T. Abbott and NOAO/AURA/NSF)
NCG 6217, a barred spiral galaxy in Ursa Minor.
A false-color image of the relatively small barred spiral (only about a third the diameter of our Galaxy), which is about 60 million light years distant. Its center is sufficiently bright to be considered an "active" galactic nucleus, and is therefore thought to contain a supermassive black hole which is stimulating radiation over a broad range of wavelengths. (NASA, ESA, and the Hubble SM4 ERO Team, apod091228)
Below, an image of NGC4565
, an edge-on spiral galaxy in Coma Berenices
, about 30 million light-years from our Milky Way galaxy. Dust scattered throughout the disk obscures its light, but the nuclear bulge is clearly visible above and below the dust lanes. A little over 100,000 light-years in diameter, NGC4565 is thought to be nearly identical to our own galaxy, and as a result it is presumed that our galaxy would look very much like this if seen from the same angle. (Bruce Hugo & Leslie Gaul, Adam Block, NOAO/AURA/NSF)
A "thin" spiral galaxy, NGC 5866
, is seen exactly edge-on, so that the lanes of gas and dust that fill its disk appear to be nearly a straight line. As remarkable as the galaxy appears, it is likely that many spiral galaxies would appear equally thin if seen from such an angle. The blue disk of young stars extends well past the thin dust lanes, while the central bulge, more orange in color, due to the older, redder stars that exist in the nucleus, extends above and below the disk. About 60 thousand light years in diameter, and 45 million light years away, in the constellation of Draco
the Dragon, NGC 5866 is about 30% smaller than our galaxy, but about equally massive, based on its rotational rate, and the dynamics of the NGC 5866 group of galaxies, of which it is the brightest member. NGC 5866 is also notable as the probable "lost" Messier object, M102
. (NASA, ESA, and The Hubble Heritage Team (STScI/AURA), W. Keel (U. Alabama), apod060612)
(Image Credit AURA, NSF, NOAO)
Galaxies are always much bigger than they appear in "normally" exposed photographs. Above and below are images of M77
, a spiral galaxy in Cetus
. The image above (rotated 45 degrees, to correspond to the orientation of the image below) shows the galaxy as usually photographed, with an unusually bright core (hence its designation as a Seyfert galaxy), and tightly wound spiral arms extending a moderate distance from the core. The image below, which uses a much longer exposure for the outer regions, shows that the actual structure is many times larger.
Although the density of stars per unit of space is lower in the outer, fainter parts of galaxies, the much larger extent of those regions makes the overall mass of the outer regions comparable to, or even larger than, the mass of the "visible" portions of the galaxy.
(Image Credit Francois and Shelley Pelletier/Adam Block/AURA/NSF/NOAO)
M83, a barred spiral galaxy in Hydra.
One of the closest galaxies, at only 15 million light years distance. (FORS Team, 8.2-meter VLT, ESO, apod030511)
, a barred spiral galaxy in the Fornax Cluster. A rotating density wave helps maintain the structure of the bar and promotes the creation of new stars, which light up the spiral arms extending from the bar. (FORS Team, 8.2-meter VLT Antu, ESO, apod030413)
Bright blue giants light up the outer arms of the strikingly barred spiral galaxy NGC 1300
. The 100,000 light-year-diameter galaxy lies a mere 70 million light years away, allowing a detailed examination of its structure, including the remarkable spiral structure near its center. (Hubble Heritage Team, ESA, NASA, apod060827)
ESO 510-13, a warped spiral galaxy seen from the side.
About 100,000 light years across, and 150 million light years away. From this direction, dust in the disk obscures the view. (Hubble Heritage Team (STScI / AURA), C. Conselice (U. Wisconsin / STScI) et al., NASA, apod030607)
M106, a spiral galaxy in Canes Venatici.
A relatively small galaxy, at only 30,000 light years across, but only 21 million light years away. Notable because it is a Seyfert galaxy, having an exceptionally bright core. (Bernie and Jay Slotnick, Adam Block, AOP, NOAO, AURA, NSF, apod030417)
NGC 1275, a galactic collision in the Perseus Cluster, about 230 million light years away.
Two galaxies, a dusty spiral near the center and an elliptical at lower left, are colliding. Each galaxy, about 50,000 light years across, is distorted by the gravity of the other one, and as clouds of gas in the galaxies collide, bursts of star formation occur. See M81 and M82: A Cosmic Train Wreck
for more about galactic collisions. (Hubble Heritage Team (STScI/AURA), M. Donahue (STScI) & J. Trauger (JPL), NASA, apod030505)
ESO 269-57, a face-on spiral galaxy in Centaurus.
ESO269-57 (the -57 refers to its declination) is a relatively large galaxy, almost 200,000 light years across. Its distance of 150 million light years means that we see it as it was 150 million years ago. (FORS1, VLT, ESO, apod030419)
Below, an image of Hoag's Object (PGC 54559
), a "ring" galaxy in Serpens
. First noticed in 1950 by astronomer Art Hoag, the physical nature of Hoag's Object was uncertain until fairly recently. It is now known to be a peculiar type of galaxy, in which a bright central nucleus consisting of ancient lower Main Sequence stars is surrounded by a ring of hot young stars, similar to those found in the arms of spiral nebulae, but of less certain origin. If not for the bluish ring, the galaxy would probably be thought to be an elliptical galaxy, or perhaps a spiral galaxy of type S0 (one in which the disk is almost too faint to notice, in comparison to the nucleus). The mechanisms which led to such a strange distribution of young stars are unknown, but probably unusual (see the next paragraph), so such galaxies are probably uncommon; but ironically, another one is visible to the upper right of the nucleus. (Given its small size, the second ring galaxy is probably five to ten times the nearly 600 million light year distance of Hoag's Object, so there is no connection between the two, save for coincidence.) Hoag's Object is about 150 thousand light years diameter, so its nucleus probably contains more than 200 billion solar masses, in a similar number of objects. Given the unusual nature of its "disk", the mass of the disk is more uncertain, but is probably about the same.
The origin of objects such as Hoag's would normally be attributed to gravitational encounters with other galaxies, as in the case of the probable interaction of M31
and M110. But there are no other galaxies visible in the vicinity of Hoag's Object. Study of other ring galaxies suggest that if Hoag's Object is the result of such a collision, it probably occurred two to three billion years ago, and the "ring" may be fragments of the other galaxy, or ongoing star formation triggered by the shock of the encounter. (R. Lucas (STScI/AURA), Hubble Heritage Team, NASA, apod040815)
Hickson Compact Group 31
, four dwarf galaxies spanning a region in Eridanus
not much larger than our galaxy -- about 150 thousand light years across -- and therefore colliding with each other, and probably fated to merge into a single galaxy within the next billion or so years. Most galaxies in collision are billions of light years away, and studying their collisions in detail is difficult; but Hickson 31 is only about 165 million light years away, and study of its star clusters and stellar birthplaces may greatly increase our understanding of the violent events which accompany galactic collisions. Among other things, it has been determined that the richest star clusters -- each with more than 100 thousand members -- are less than 10 million years old, a mere blink of an eye on cosmic time scales. (NASA, ESA, J. English (U. Manitoba), and the Hubble Heritage Team (STScI/AURA);
Acknowledgement: S. Gallagher (U. Western Ontario), apod100222)
Stephan's Quintet, a physical and optical grouping of galaxies
(NASA, ESA, Hubble SM4 ERO Team, apod090911)
NGC6240 -- Two galaxies in collision
(NASA / JPL-Caltech / STScI-ESA / S. Bush, et al. (Harvard-Smithsonian CfA), apod090618)
Below, a type II supernova observed in 1999, in NGC 3184, a spiral galaxy in Ursa Major.
The bright dot just above the center of the galaxy is the supernova. (This is not
the bright dot near the top of the galaxy, which is a nearby star, in our own galaxy.) NGC 3184 is a type Sbc galaxy about 50,000 light years across, or about half the size of our galaxy, and 40 million light years distant. Another image, showing the galaxy after the supernova faded away, is shown on this object's NGCIC page. (Al Kelly (JSCAS/NASA) & Arne Henden (Flagstaff/USNO), apod000920)