Astronomy/Galaxies
Galaxies was previously the Astronomy topic for the 2010 season. Specific types of galaxies such as active galaxies and starburst galaxies have been Astronomy topics in more recent years, with active galaxies being the focus of the event in 2011 and starburst galaxies being the focus in 2019.
Galaxies
A galaxy is a gravitationally bound group of stars, dust and stellar remnants. In Astronomy, teams are tested over their knowledge of galaxies, various DSOs (deep sky objects), and mathematical relationships.
Galactic Structures
There are four main structures that may be found in galaxies: the disk, the halo, the bulge, and the nucleus.
Disk
This is the flattened area of a galaxy that surrounds the galactic bulge. It contains a majority of the luminous stars and interstellar matter (gas and dust in and amongst the stars). It contains both young and old stars, but has the highest quantity of young stars of these structures and is undergoing star formation. Stars move in circular orbits in the galactic plane of the disk.
Halo
This is the spherical area all around the galaxy. It is relatively dim, and contains only old stars. It does not contain interstellar medium, and is not undergoing star formation. The galactic halo contains a majority of the globular clusters. It appears reddish in color. Stars in the halo move in random orbits (in three dimensions).
Bulge
This is the area of a galaxy where the disk bulges in the center. It contains young and old stars, with a higher number of old stars toward the outer region of the bulge. It has ongoing star formation, especially toward the center, and has interstellar medium, in higher quantity toward the center. It appears yellow-white. Stars in the bulge have random orbits (in three dimensions)
Nucleus
This is the dense center of the galaxy, often contained within the bulge. It has a high density of stars and interstellar matter.
Hubble Classification
Edwin Hubble (American astronomer) was the first astronomer to categorize galaxies in a comprehensive manner, based heavily on their shape and structure. He classified normal galaxies into four groups: Spiral Galaxies, Barred Spiral Galaxies, Elliptical Galaxies, and Irregular Galaxies.
Spiral Galaxies
Spiral galaxies are denoted by the letter "S" in the Hubble classification scheme. Spiral galaxies consist of an inner core of old stars moving slowly and outer arms of faster-moving younger stars. Spiral galaxies contain a flattened disk with apparent spiral arms, a dense bulge, and a halo. The bulge has the highest stellar density (stars per unit volume) in spiral galaxies. The disks of spiral galaxies are rich in interstellar matter. A high amount of star formation occurs in their spiral arms, which contain emission nebulae. Newly formed O and B-type stars cause the spiral arms to appear bluer than the rest of the galaxy. Spiral galaxies are classified into three subtypes, Sa, Sb, and Sc. Some major differences between Sa spiral galaxies and Sc spiral galaxies are listed in the table below, with Sb being an intermediate between the two in all categories.
| Sa | Sc | |
|---|---|---|
| Appearance | More tightly wrapped spiral arms, almost circular in appearance. | More open, loose, poorly defined spiral arms. Displays knotty (or clumped) structure. |
| Bulge | Typically the largest bulge of the three types. | Typically the smallest bulge of the three types. |
| Interstellar Matter | Least quantity of interstellar matter of the three types. | Highest quantity of interstellar matter of the three types. |
Barred Spiral Galaxies
Barred Spiral Galaxies are a variation of Spiral Galaxies, and are denoted by "SB" in the Hubble Classification Scheme. They have very similar properties to spiral galaxies, but have a horizontal bar of stars protruding out from the galactic core into the disk, and the spiral arms project from the ends of the bar. The subclasses (SBa-SBc) have the same properties as spiral galaxies in their classification.
Elliptical Galaxies
Elliptical galaxies are denoted by the letter "E" in the Hubble Classification Scheme. Elliptical Galaxies are generally older and larger than spiral galaxies. They display little structure of any type, with a dense nucleus in the center and halo surrounding it. They tend to not display a galactic bulge or disk, and it is very weak if they do. Elliptical galaxies contain very little cool interstellar matter, and there is no evidence of ongoing star formation or young stars. Elliptical galaxies contain mostly old, red stars. However, elliptical galaxies do contain large amounts of very hot interstellar gas.
Elliptical galaxies have a huge variation. They range from huge in size (giant ellipticals) to very small in size (dwarf ellipticals). In the Hubble Classification Scheme, they are ordered in terms of eccentricity. E0 ellipticals are perfectly circular, and E7 ellipticals cigar-shaped.
Lenticular Galaxies
This is a type of galaxy between an elliptical galaxy and a spiral galaxy. It contains a thin disk and bulge, but no gas or spiral arms. These galaxies are classified as S0 galaxies (or SB0 if they have a bar).
Irregular Galaxies
Irregular galaxies are denoted by "Irr" in the Hubble Classification Scheme. Their appearance does not fit them into any other category, and they tend to exhibit little to no internal structure. They tend to have lots of interstellar matter and young, blue stars.
Irregular galaxies also have subclasses. The more common Irr I irregulars look more like misshapen spiral galaxies. The much rarer Irr II galaxies have a much more irregular shape, and commonly have an explosive or filamentary appearance.
Formation & Evolution
Studies on galaxy formation and evolution, unlike star formation and evolution, are still very young and in the early stages. There are some widespread theories that have gained popularity, however, of galaxy formation and evolution.
Formation
Galaxies likely are a result of slight differences in density in the early universe. Places with higher density would get matter pulled toward them through gravity, while places with lower density would have matter pulled away from them. These galaxies may have merged and grown to form the large galaxies we see today.
Evolution
Galaxies can evolve in two ways. They can evolve passively (without outside influence of other galaxies) or, much more commonly through interaction.
Passive Evolution
A galaxy's overall color, composition, and appearance will change as stars evolve and interstellar matter gets used up, and recycled through star death. The already red elliptical galaxies will likely get redder and fainter. The blue spiral and irregular galaxies will stay blue as long as gas remains are available for them to reform.
Interaction
Galaxies can interact to change in their entire internal structure, which can trigger star formation or causing activity in the galactic nucleus.
Galactic Interactions
Galaxies can interact with each other gravitationally in many ways. The most common is a near miss, where two galaxies pass each other nearly. The matter in both galaxies is gravitationally pulled together and the two galaxies begin to change structure, even from afar. This likely results in the galaxies returning together to merge. If the merger of galaxies was between a small and large galaxy, it is called galactic cannibalism, and the structure of the larger galaxy is likely to remain mostly unaffected. On the other hand, if the merger is between two galaxies of similar size, it results in a dramatic change in both galaxies (if both were spirals, it will likely end in an elliptical galaxy).
Starburst Galaxies
- Main article: Astronomy/Starburst Galaxies
Starburst galaxies are typified by significantly greater levels of star formation than normal galaxies. They are often normal galaxies that have collided with each other, causing gravity ripples and star formation. Past DSO lists have included starburst galaxies such as M82, M84, M101, the Cartwheel Galaxy, and C153.
Dark Matter
Measurements of mass in space can sometimes not match up with the amount of mass we can actually see, and by a significant amount. One theory to explain this extra matter in the universe classifies the extra matter as Dark Matter, because we cannot see it in any wavelengths. We can only measure dark matter by the gravitational effect it has on normal matter. Otherwise, however, dark matter does not appear to interact with normal matter. While dark matter is dispersed throughout the universe, it specifically can affect how matter in galaxies interacts, and it can affect the motion of the galaxy.
Galaxy Groups and Clusters
Galaxy Groups
Galaxies are usually located close to other galaxies. A galaxy group is the smallest group classification, and it refers to a group of about 30-50 galaxies. The Milky Way is located in the Local Group, along with the Andromeda and Triangulum Galaxies.
Galaxy Clusters
Galaxy clusters are larger than galaxy groups, containing hundreds to thousands of galaxies. They are among the largest known gravitationally bound systems in the universe. Galaxy clusters are massive systems, that get around 1% of their mass from the galaxies in them. Around 9% of the mass comes from extremely hot gas (typically millions of Kelvins) very bright in X-Rays. Clusters typically consist of around 90% dark matter, observed through gravitational interactions such as gravitational lensing (the bending of spacetime, causing light to be bent around an object, and a background object to appear magnified, distorted, in different areas of the sky, and/or multiple times.
Galaxy clusters are largely closed systems, and can be used to observe processes that happen in the universe (such as nucleosynthesis) on a smaller scale. Additionally, galaxy clusters tend to evolve very slowly, providing a probe to observe galaxy evolution.
Galaxy cluster collisions are some of the most energetic events in the universe.
Galaxy Clusters are formed because of very slight variations in density in the early universe. As the universe continued to expand, and the space between objects increased, more and more matter was attracted toward these denser spots, eventually becoming a galaxy cluster.
Galaxy Superclusters
The largest classification is that of galaxy superclusters, which are groups of other groups and clusters. The are massive, and span hundreds of millions of light years. They typically contains 3-10 galaxy clusters. The Milky Way is located in the Virgo (or Local) Supercluster.
Deep Sky Objects
- See Astronomy/DSOs for more information.
One of the topics in 2021 is galaxy formation and evolution; accordingly, many of the current DSOs are galaxies or galaxy clusters/groups. NGC 2623, M87, and DLA0817g are galaxies, while MACS J0717.5+3745, MACS J1149.5+2223, the Bullet Cluster, and JKCS 041 are groups or clusters of galaxies.