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Astronomy

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Astronomy Event Links
Forum Thread: 2009 2010

Test Exchange: 2009

2008/2009 Topic : Variable Stars

1 Variable Stars
- 1.1 Intrinsic Variable Stars
- 1.2 Extrinsic Variable Stars
2 Other Topics
- 2.1 This Year's DSO's

Variable Stars

A variable star is defined as a star that undergoes changes in luminosity. In Science Olympiad, you will be tested on your knowledge of various DSOs (Deep Sky Objects), types of variable stars, and other related things.

Variable stars are generally placed into two categories; Intrinsic variable stars and Extrinsic variable stars.

Intrinsic Variable Stars

Intrinsic Variable Stars are stars that change in luminosity due to physical changes in the star; there are three types of Intrinsic Variable Stars.

The first type is pulsating variable stars. These are stars that periodically swell and shrink. Pulsating variable stars that have short periods are called Cepheids; cepheids usually have more regular periods than Longer Period Variables. Important examples include the Delta Cepheid variables, W Virginis variables, RR Lyrae variables, and Beta Cephei variables.

The other type of pulsating variable are long period variables. They have longer periods than cepheids (such as a year or longer) and have a much less regular period. These variables are also sometimes bigger and cooler than cepheids. A good example is the Mira Variables. Long period Variables can be split into three subgroups. The First is Irregular variables; these are usually red super giants and have almost no regular period. Second is the Semi regular variables, these sometimes go through regular periods, then change and go on an un-regular period. And third are Nonradial pulsators; variables that change to shapes other than the sphere to change magnitude.

The second major type of intrinsic variable star are eruptive variable stars. These are stars are mostly pre-main sequence stars, but an exceptional few main sequence stars are eruptive variables. Pre-main sequence (also called protostars) are stars that have not completed the process of becoming a main-sequence star from a gas nebula and are not yet condensed. So while condensing they change in magnitude. Eruptive Main-sequence variables are usually extremely larger or extremely smaller than the average main sequence star. Some eruptive variables are red giants since they easily lose their gases. The last type of eruptive variable star is binary eruptive stars. These stars flare up and can remain that way for 1-4 years. Important eruptive variable stars are the Orion Variables, the Wolf-Rayet variables, and RS Canum Venaticorum variables.

The third major type of an intrinsic variable star is a cataclysmic or explosive variable star. The most dramatic type of these variable stars are called Supernovae. This only occurs in extremely massive and old stars. The outer layers of the star are expelled at high speeds creating a supernovae remnant or nebula. A white dwarf or pulsar is usually left behind. The second type of these variables are called Novae. They are dramatic explosions caused by a pair of close binaries but don’t cause the total destruction of the star. An other type are dwarf novae and are very similar to novae. Dwarf novae are just two binary white dwarfs that regularly have outburst. The fourth and final type of intrinsic variable are Z Andromedae Variables. These are a less common type of variable and are caused by a double star system containinga red giant and a hot blue star; enclosed in a cloud of dust and gas.

Extrinsic Variable Stars

Extrinsic variable stars can stars that change in luminosity due to external changes; the two major types are rotating and eclipsing.

Rotating variable stars are stars that change in luminosity due to the rotation of the star. This can happen because of such things as a sunspot on the surface as it rotates. This can also occur when two close binary stars change shape due to their mutual gravity. Fluctuations in magnetic fields can cause slight change in magnitude.

The second type of extrinsic variable star are eclipsing variable stars. These are caused when stars in double star systems eclipse, causing a lowing of magnitude. However, this variation can only be viewed from certain angles. Planetary transits can also cause a very small variation in luminosity only viewed with very accurate instruments.

Galaxies

A galaxy is a gravitationally bound group of stars, dust and stellar remnants. In Astronomy, you will be tested over your knowledge of galaxies, various DSOs (deep sky objects), and mathematical relationships.

This year's galaxies fall into two categories: normal galaxies, and starburst galaxies.

Normal Galaxies

Normal galaxies are classified by the Hubble system as being either elliptical or spiral. Elliptical Galaxies are generally older and larger than spiral galaxies. Spiral galaxies consist of an inner core of old stars moving slowly and outer arms of faster-moving younger stars. They also have a separate classification, as some spirals are barred, or they have a horizontal bar of stars protruding out from the galactic core. The normal galaxies include the Milky Way Galaxy, the Andromeda Galaxy and the Triangulum Galaxy.

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. The starburst galaxies in this year's DSOs include M82, M84, M101, the Cartwheel Galaxy, and C153.

Deep Space Objects

Although the focus of this year's event is galaxies, many of the DSOs are other stellar objects including: M15 the globular cluster, Sag A* the black hole, Epsilon Aurigae the eclipsing binary, G1 (Mayall II) the globular cluster, NGC 604 the stellar nursery, and M33 x-7 the x-ray binary. Most of these are prominent objects located in the other galaxies studied.

This Year's DSO's

Epsilon Aurigae*
- AAVSO: Epsilon Aurigae
Milky Way Galaxy
- NASA: Our Milky Way Gets a Makeover
Sagittarius A*
- Chandra Photo Album: Sagittarius A*: 06 Jan 03
Andromeda Galaxy (M31)
- NASA APOD: November 14, 1999 - M31: the Andromeda Galaxy
Cartwheel Galaxy
- Chandra Photo Album: Cartwheel Galaxy: 11 Jan 06

-*is part of a special viewing campaign this year and will be included up to at least 2011.

Other Information

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

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 contain stations or not. 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.