Astronomy

In Astronomy, teams answer questions on math and physics relating to the year's topic. For 2012, the topic of Astronomy will be stellar evolution and type Ia supernovae. Some questions pertain to specific DSOs on the year's DSO list.

''This page is for the general Astronomy information. For specific topics, see the following pages:

H-R Diagram


The Hertzsprung-Russell diagram relates the absolute magnitudes and luminosities of stars with their spectral types and temperatures. They are especially important in understanding stellar evolution. Although some diagrams may have more characteristics labeled on them than others, including characteristics not listed above like Color Index, they all have basically the same shape. Here, a basic introduction to the diagram and its usefulness will be given.

First, the H-R Diagram reveals key relationships in characteristics of stars. The first and most apparent of these is in the main sequence, which contains all of the stars that form a band in the middle of the diagram. The vast majority of stars fall within this band, including the Sun. Also, giants are found in a group above the main sequence, and white dwarves have their own conglomerate on the lower-left part of the diagram. The fact that these stars occupy distinct sections shows how a star's age can change its physical properties.

Another use of the H-R Diagram is that it can predict the location of a new, previously unknown star based on certain observations. For example, say a new star was discovered that had a temperature of 10,000 K and was known to be part of the main sequence. By looking at the diagram, you can predict that the star will have a luminosity of between 100 to 1000 solar luminosities.

Determining Distances
A 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
''This section deals with the uses of Cepheids and RR Lyrae in determining distances. For information about their physical properties, please see Astronomy/Variable Stars''

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. Cepheids typically have periods of about 1 to 50 days. 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. RR Lyrae stars typically have shorter periods than Cepheids - usually less than one day. 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. However, this makes them very useful in determining distance, because once an RR Lyrae star has been found, one only needs to know the apparent magnitude in order to put it into the distance modulus equation and find distance. 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. Several of these equations can be found in the [[Media:Formula Sheet.pdf|Astronomy formula sheet]].

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.

Distance Modulus
The distance modulus equation is also very important. It relates an object's distance with the difference between the apparent magnitude (m) with the absolute magnitude (M). This difference is known as the distance modulus.
 * $$5(log(d)-1)=m-M$$ where d is in parsecs

This equation can be written in many different ways so that different values can be found, but the essential purpose of the formula remains the same. A good way to practice using this equation before the competition is to take the apparent magnitude and approximate distance to a DSO and use them to find the absolute magnitude. This experience will be a time-saver if you have to use it during the test.

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
For information regarding stellar evolution, please see the Stellar Evolution main page.

Variable Stars
For information regarding variable stars, please see the Variable Stars main page.

The Competition
The competition usually consists of a test, which may be PowerPoint or station-based. Each team member may bring a laptop or a binder. It is advisable to bring as much information as you can, as a wide breadth of material may be covered. Organize your information so that it is easily referenced during the exam. Most Astronomy tests include mathematical computations, so it is important to have a calculator and a formula sheet ready.

Laptop or Binder?
The question of using laptops or binders as resources has plagued Science Olympians for years. In the end, it comes down to personal preference, and you may have to toy with combinations of two laptops or binders or one of each to see what works best for you and your partner. Here is a list of advantages and disadvantages to help you get a feel for each resource type.


 * Binder
 * Advantages
 * You can take things in and out of the rings
 * The process of organizing your binder helps you retain information
 * You have a hard-copy of all of your information
 * You can write notes on your papers
 * Disadvantages
 * More limited in terms of data storage
 * If you're not very familiar with your binder, it can be hard to find certain information
 * Large binders use up LOTS of paper
 * Laptop
 * Advantages
 * Much higher capacity for data storage
 * Easier to carry
 * Availability of Find/Search functions
 * Process of opening your files before competition makes you look pro
 * Disadvantages
 * Harder to look at multiple pages at once
 * No hard-copy of the information (unless you use one binder and one laptop)
 * More difficult to write personal notes

Useful Resources

 * [[Media:Formula Sheet.pdf|Formula Sheet for Math Portion of Astronomy]] for the mathematical section
 * Good overall site for the basics of Astronomy
 * American Association of Variable Star Observers
 * Images from the Chandra X-Ray Observatory
 * NASA Astronomy Picture of the Day
 * Reach for the Stars for some sample pictures
 * Basic note sheet for the 2010-2011 DSOs
 * SIMBAD Astronomy Database for DSOs
 * NASA Space Math provides work sheets for a wide variety of Astronomy math problems
 * List of Messier objects Helpful for identification