The Spiral galaxy is most peoples idea of the classic galaxy. Wonderful images of the Whirlpool and Pinwheel galaxies from Hubble are perfect examples. But disk type galaxies encompass many differing styles.
Early on in the universe, galaxies started as irregular clouds of gas and dark matter, with few, if any, stars. As it gains mass through the process of merging with other proto-galaxies, the dark matter does not interact with the gas in the cloud (except through gravity) so the gas starts to contract. The pressure in the cloud increases leading to star formation. Rather like ballet dancers pulling in their arms as they spin, the cloud's rate of rotation increases as it collapses producing a thin, rapidly rotating disk. As stars form, and the proto-galaxy possibly gets an active nucleus at its center, the massive amounts of radiation help to stop the overall contraction of the cloud allowing further star formation. The dark matter, probably forming a halo around the galaxy, can also slow the contraction through gravity.
Disk galaxies comprise the following basic types:
• Spiral Galaxies
• Barred and Intermediate Barred Spirals
• Intermediate barred spirals
• Lenticular galaxies that are intermediate between disk and elliptical galaxies.
There is further delineation depending upon how strong the spiral arms appear, how tightly they are curled, and the brightness of the central core, as described in the following table. Barred
Early on in the universe, galaxies started as irregular clouds of gas and dark matter, with few, if any, stars. As it gains mass through the process of merging with other proto-galaxies, the dark matter does not interact with the gas in the cloud (except through gravity) so the gas starts to contract. The pressure in the cloud increases leading to star formation. Rather like ballet dancers pulling in their arms as they spin, the cloud's rate of rotation increases as it collapses producing a thin, rapidly rotating disk. As stars form, and the proto-galaxy possibly gets an active nucleus at its center, the massive amounts of radiation help to stop the overall contraction of the cloud allowing further star formation. The dark matter, probably forming a halo around the galaxy, can also slow the contraction through gravity.
Disk galaxies comprise the following basic types:
• Spiral Galaxies
• Barred and Intermediate Barred Spirals
• Intermediate barred spirals
• Lenticular galaxies that are intermediate between disk and elliptical galaxies.
There is further delineation depending upon how strong the spiral arms appear, how tightly they are curled, and the brightness of the central core, as described in the following table. Barred
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Science - Galaxies
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Disk Galaxies
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| Class | Description |
| Sa (or SAa) | Has tightly wound spiral arms around a bright core. |
| Sb (or SAb) | Loose spiral arms, and a dimmer core than type Sa. |
| Sc (or SAc) | Even looser spiral structure, and a dimmer core than type Sb. |
| Sd (or SAd) | Very loose spiral structure, with a very dim core. The spiral arms evidence gas and dust with new stars forming. |
| Sm (or SAm) | Similar to Sd, but with virtually no discernable central bulge. |
| SBa-m | As above for Barred Spiral galaxies. |
| SABa-m | As above for Intermediate Barred Spiral galaxies. |
| (r) | Indicates the presence of a ring structure |
| (s) | Indicates the absence of a ring structure |
| (rs) | Indicates a very weak ring structure |
| SA0 | A Lenticular galaxy. |
| SB0 | A Barred Lenticular galaxy. |
spirals have similar designations using classifications SBa, SBb, SBc and SBd. Some galaxies are intermediate; for example, a galaxy between SBb and Sbc would de designated SBbc.
The oldest stars are in the central bulge and in the surrounding globular star clusters, while new star formation generally occurs in the spiral arms. Astronomers believe that only about 20% of all galaxies are discs, but they tend to be much brighter than elliptical galaxies.
Galaxies tend to start off as disk galaxies, either spiral or lenticular, with a thin, rapidly rotating disk surrounding a central core or "bulge" with a halo around it. Star production begins in the central core and halo region, extending out into the disk over time. The stars in the disk rotate around the galactic center. In theory, apart from very close to the center, the further out you go from the center, the slower the stars should move, which would destroy any spiral arm structure within a few revolutions. In practice, due to the invisible dark matter spread through and, mainly, around the galaxy, the stars in the disk tend to move at more or less the same speed regardless of distance from the center. The Sun is located in the disk, about two-thirds of the way out from the center.
The very earliest stars, called Population III stars, would have been large and hot, and would have burned out very quickly. No Population III stars have ever been identified. The oldest we see today are Population II, found in the central bulge and globular clusters. Our own Sun is a Population I star, which is relatively young at around 4.5 billion years old. Eventually the rate of new star production slows as most of the gas and dust is already incorporated into stars, many of which have evolved into white dwarfs or neutron stars or black holes. Thus the color of the galaxy also evolves starting at the blue end of the spectrum and becoming redder. Of
The oldest stars are in the central bulge and in the surrounding globular star clusters, while new star formation generally occurs in the spiral arms. Astronomers believe that only about 20% of all galaxies are discs, but they tend to be much brighter than elliptical galaxies.
Galaxies tend to start off as disk galaxies, either spiral or lenticular, with a thin, rapidly rotating disk surrounding a central core or "bulge" with a halo around it. Star production begins in the central core and halo region, extending out into the disk over time. The stars in the disk rotate around the galactic center. In theory, apart from very close to the center, the further out you go from the center, the slower the stars should move, which would destroy any spiral arm structure within a few revolutions. In practice, due to the invisible dark matter spread through and, mainly, around the galaxy, the stars in the disk tend to move at more or less the same speed regardless of distance from the center. The Sun is located in the disk, about two-thirds of the way out from the center.
The very earliest stars, called Population III stars, would have been large and hot, and would have burned out very quickly. No Population III stars have ever been identified. The oldest we see today are Population II, found in the central bulge and globular clusters. Our own Sun is a Population I star, which is relatively young at around 4.5 billion years old. Eventually the rate of new star production slows as most of the gas and dust is already incorporated into stars, many of which have evolved into white dwarfs or neutron stars or black holes. Thus the color of the galaxy also evolves starting at the blue end of the spectrum and becoming redder. Of
course, this process takes billions of years. A disk galaxy can absorb a much smaller galaxy and retain its structure, as the Milky Way has done in the past. If two disk galaxies of similar, or near similar, size collide, it is highly unlikely that the result would maintain any spiral structure. Gravitational interaction would cause random changes in the motions of stars leading to a galaxy much more like an elliptical or irregular galaxy. Thus we would expect to find that elliptical galaxies are more common than disk galaxies, and generally much larger, which is the case.
M33
M33, the Triangulum galaxy, is a type SA(s)cd spiral galaxy. Confusingly, it is sometimes known as the Pinwheel, but should not be confused with M 101, below. It is approximately 920 kpc (or three million light years) away, 15 kpc or 50,000 light-years across, with a mass estimated at around 50 billion solar masses. It is the third largest galaxy in our Local Group after Andromeda and the Milky Way.
M101
M101, the Pinwheel galaxy, is a face-on type SAB(rs)cd (intermediate Barred Spiral, between types "c" and "d", with a weak ring structure) galaxy. It is about 7.18 Mpc or 23.4 million light years away. There is more information in the section on the Hubble Space Telescope.
M33, the Triangulum galaxy, is a type SA(s)cd spiral galaxy. Confusingly, it is sometimes known as the Pinwheel, but should not be confused with M 101, below. It is approximately 920 kpc (or three million light years) away, 15 kpc or 50,000 light-years across, with a mass estimated at around 50 billion solar masses. It is the third largest galaxy in our Local Group after Andromeda and the Milky Way.
M101
M101, the Pinwheel galaxy, is a face-on type SAB(rs)cd (intermediate Barred Spiral, between types "c" and "d", with a weak ring structure) galaxy. It is about 7.18 Mpc or 23.4 million light years away. There is more information in the section on the Hubble Space Telescope.
