Astronomy

Past Topics:

Variable Stars

Galaxies

This year's topic is centered around quasars, AGNs (active galactic nucleus), galaxy clusters, and groups of galaxies.

This Year's DSOs (2010-2011)
-**Epsilon Aurigae is part of a special observing campaign through 2011 and will also be included in the DSO list for the 2011/2012 season.
 * Epsilon Aurigae**
 * AAVSO: Epsilon Aurigae
 * NGC 6240
 * Chandra: NGC 6240
 * 3C 321
 * Chandra: 3C 321
 * Centaurus A (NGC 5128)
 * SEDS: Centaurus A
 * Solstation.com: Centaurus A
 * Stephan's Quintet
 * Chandra: Stephan's Quintet
 * NASA APOD: Stephen's Quintet
 * MACSJ0717.5+3745
 * Chandra: MACSJ0717.5+3745
 * Bullet Cluster (1E 0657-56)
 * Chandra: Bullet Cluster
 * NASA APOD: Bullet Cluster
 * Perseus A (NGC 1275)
 * NASA APOD: Perseus A
 * Hubble Heritage: Perseus A
 * SN 2006gy
 * Chandra: SN 2006gy
 * NASA APOD: SN 2006gy
 * SN 1996cr
 * Chandra: SN 1996cr
 * NGC 4603
 * NASA APOD: NGC 4603
 * HyperPhysics: NGC 4603
 * NGC 7771
 * NASA APOD: NGC 7771
 * NGC 2623
 * NASA APOD: NGC 2623
 * Hubble: NGC 2623
 * JKCS041
 * Chandra: JKCS041
 * Messier 77 (NGC 1068)
 * Chandra: Messier 77
 * Universe Today: Messier 77
 * H2356-309
 * Chandra: H2356-309

Other Information
Although these topics are the focus of this year's competition, there are other topics which you will need to be familiar with.

Galaxies
Galaxies make up a majority of the Astronomy event not covered by the DSO list. Many of the DSO's are galaxies with interesting characteristics. Since this year's competition focuses on active galaxies, many of the DSO's are active.



AGN's and Quasars
AGN's, or Active Galactic Nuclei, are defined as galactic nuclei that emit more electromagnetic radiation than a normal galaxy. This radiation is emitted in a large jet in one direction. In addition, a torus of gas forms around the nucleus perpendicular to the direction of the jet, which can obscure some parts of the galaxy from observers, changing their visual characteristics.

The six main characteristics that define AGN's are: compact angular size, high luminosity, continuum radiation (all types of radiation in the spectrum are emitted), emission lines, variability of emission, and strong radio emission. Quasars, or quasi-stellar radio sources, are similar, as they are galaxies that contain active nuclei. This makes them slightly different than AGN's, but closely related.

Seyfert Galaxy

Seyfert Galaxies are subclasses of active galaxies classified by emission lines of ionized gas. The two main classifications are Type 1 and Type 2. Type 1 Seyferts emit narrow and broad spectral lines, while Type 2 Seyferts only emit narrow lines. Some galaxies can also be classified as numbers between 1 and 2, like 1.5, depending on the relative sizes of the lines. Perseus A is an example of a Type 1.5 Seyfert galaxy.



Other classifications

AGN's and quasars are mainly classified based on the angle that the object is viewed from by an observer on Earth. This is because the angle of the electromagnetic radiation emitted from the nucleus can change how visible the object is. If the emission jet faces away from the observer, the object will appear less active, and vice versa. The diagram at left shows how viewing angle correlates to type of AGN.

AGN's are classified into two large groups, Radio-Quiet AGN's and Radio-Loud AGN's. In Radio-Quiet AGN's, the large jet of radiation faces away from the observer. Seyfert galaxies are Radio-Quiet AGN's. In contrast, Radio-Loud AGN's occur when the jet of radiation faces the observer, and much more activity is apparent. In some cases, the radiation is pointed directly at earth, so an observer can see the radiation jet head-on. There objects are known as Blazars. BL Lacertae and OVV (optically violent variable) Quasars, which are both very variable (OVV Quasars more so), are both Blazars.

Black Holes
Black holes are supermassive objects from which nothing can escape. Since light cannot be emitted, they appear as black spots in space. They can also alter the appearance of surrounding objects because of the high gravitational pull.

Mid-mass black holes, or stellar black holes, have masses at 1.4 to 20 times the Sun. They are very dense and result from the collapse of a large star. Intermediate-mass black holes are thought to be at the center of globular clusters, and they are larger than mid-mass black holes. The largest black holes, supermassive black holes, are usually the centers of galaxies. The center of the Milky Way, Sagittarius A*, is thought to be a supermassive black hole. AGN's are usually supermassive black holes. Supermassive black holes can be over one billion solar masses and less dense than water.

Supernovae
A supernova is, in short, the explosion of a star. This term can apply to several different types of explosions, though, and so, like many other astronomical terms, there are classifications. Type Ia supernovae are explosions of white dwarves once they reach the end of their life. Type Ib and Ic supernovae are formed when a large star is stripped of its outer hydrogen layers. The Type I supernovae are generally associated with binary systems. Type II supernovae are explosions of giants stars that occur when the star gets too massive.

Galaxy Groups and Clusters
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 are slightly larger than groups. Although they seem to be held together by gravity, there is no set structure to galaxy clusters. The largest classification is that of galaxy superclusters, which are groups of other groups and clusters. The Milky Way is located in the Virgo (or Local) Supercluster.



Binaries
A binary star is a system of two stars that orbit a common center of gravity, or barycenter. These systems make up nearly 80 percent of all stars in the Milky Way. Binaries and other multiple-star systems can be visual, eclipsing, astrometric, spectroscopic, or a combination of these.


 * Visual binaries appear to the unaided eye to be one star, but can be seen as two through a telescope. An example is Polaris, which is made up of Polaris A ( which is two more stars in itself) and Polaris B.


 * Eclipsing binaries appear to be single stars through a telescope; however, by measuring the brightness of an eclipsing binary, one can determine that the brightness changes over time. This change of brightness is because the plane of these stars' orbit lies along our line of sight. When one star passes in front of the other, it appears as though the "star" gets dimmer. Thus, their light curves reveal occasional dips in luminosity between constant periods due to this eclipse. Epsilon Aurigae, one of the DSO's, is a special eclipsing binary because one of its components is surrounded by a mysterious dust cloud.

Determining Distances
Another large part of the Astronomy event is being able to determine distances to objects in space from Earth. Often a question will give certain information and the participant will have to interpret and use the information to find the distance, luminosity, or some other characteristics of the object in question.

Cepheids and RR Lyrae


Cepheids and RR Lyrae are two types of variable stars that are especially good for finding distances to galaxies or other groups of stars because they have direct correlations between luminosity and period. In both Cepheids and RR Lyrae, the longer the period, the higher the luminosity. Type I Cepheids, or Classical Cepheids, are brighter, newer Population I stars (see section about stellar populations below for an explanation). Type II Cepheids are similar to Type I in terms of the relationship, but they are smaller, dimmer Population II stars. These are also called W Virginis stars. RR Lyrae are different from Cepheids in that they are older and fainter than Cepheids. They have masses about half that of our Sun, and are Population II stars. Also, the luminosity does not increase as much to a change in period, as most RR Lyrae have absolute magnitudes close to 0.75. Therefore, they are only useful in our galaxy and the one closest to us, Andromeda. RR Lyrae have been linked to globular clusters, since most variable stars in globular clusters are RR Lyrae. They are named after the original RR Lyrae in the constellation Lyra.

These variable stars are useful in calculations because once the period is found, the luminosity can be calculated or determined through the use of a period-luminosity graph. Then, through other formulas, the distance can also be determined. This gives them the use as "standard candles" in galaxies relatively close to ours in our universe. NGC 4603, one of the listed DSO's, is the furthest galaxy that a Cepheid has been used to calculate distance at 108 million light years away.

Distance Equations
There are many equations that are used to find distances to objects in space. The luminosity distance equation uses the relationship between absolute and apparent magnitude to determine the distance. This and other equations can be found in the formula sheet link at the bottom of this page.

Triangulation is often used to determine distances. This method is based on parallax shifts, apparent changes in a star's location when viewed from different locations. The parallax of a star is one-half the angular shift produced over 2 AU, or six months. In short, it is the angle subtended by 1 AU. The parallax decreases as distance increases. A star's distance in parsecs (one arcsecond) is equal to 1\parallax. Parallax can only be used to measure stars up to 1000 parsecs away.

Hubble's Law

Hubble's Law uses the fact that objects in space are receding from us to determine distance. Edwin Hubble found that the recessional velocity is proportional to the distance away an object is and created an equation, $$v=H_oD$$, where v is the recessional velocity, $$H_o$$ is Hubble's constant, and D is the distance. The exact value of Hubble's constant is disputed, but most values are about 70.

The value of v is found by looking at an object's spectrum. The recessional velocity is the redshift multiplied by the speed of light, and in order to find redshift, a spectrum must be used. Redshift is how much a spectrum shifts toward the red side of the spectrum due to recession. Redshift, or Z, is found by dividing the change in wavelength of the spectrum by the wavelength the object was expected to have.

Stellar Populations
Populations of stars are classified by their metallicity, or by how much heavy metals a star has. Population I has the greatest concentration of metals, and most of them are relatively new stars that have taken metals expelled from other stars. The Sun is included within this group, as are many stars in the outer reaches of our galaxy. Population II has some heavy metals, but not as much as Population I, as they are older and did not benefit from as much metal dust as newer stars did. Stars in globular clusters and near the core of our galaxy belong to this population. Smaller galaxies also have more stars in this population. There is also a hypothetical Population III consisting of the very first stars with little to no metal content, as they did not exist near the beginning of the universe. They did not last very long, but helped the metals to form for the later populations.

Stellar Life Cycle
The life cycle differs between stars depending on their mass. Normal-mass stars begin in stellar nurseries, and some matter condenses to create a protostar. This gains more mass until fusion begins, when it becomes a main-sequence star. Then, as it uses up its store of energy, it grows to be a giant star by the end of its lifetime. Once it uses its entire store, it collapses into a planetary nebula and later a white dwarf. Larger stars are similar, except they begin with more mass and grow to supergiants. At the end of their lifetime, they can explode in a massive explosion known as a supernova and/or collapse into a neutron star or a black dwarf.

The Competition
The competition usually consists of a test, which may or may not include numerous stations. Each team member can bring a laptop or a binder to put their information in, so bring as much as you may need, as there is a wide variety that can be asked. The test usually has many questions regarding mathematical computations, so it is important to have a calculator and a formula sheet ready.

Useful Resources

 * American Association of Variable Star Observers
 * [[Media:Formula Sheet.pdf|Formula Sheet for Math Portion of Astronomy]] for the mathematical section
 * Reach for the Stars for some sample pictures
 * Scioly Test Exchange