29.2 Measuring the Stars

29.2 Measuring the Stars

Groups of Stars

Constellations

·  Groups of stars that form animals, everyday objects and mythological characters in the sky

·  There are 88 constellations named by ancient people

·  Some are visible throughout the year depending on your location

·  Circumpolar constellations appear to rotate around the North Pole.

·  In the northern hemisphere, the Big Dipper is a circumpolar constellation

·  Some constellations are seen in the northern hemisphere or southern hemisphere, therefore constellations are classified as spring, summer, fall, or winter constellations.

·  Zodiac constellations can be seen in both the northern and southern hemispheres because they lie in the elliptical plane along the same path where planets are seen. They were used by ancient people to determine planting seasons, harvest time and ritual celebrations.

Star clusters

·  A group of stars that are gravitationally bound to each other

·  Stars appear next to each other because the human eye cannot distinguish how far or near the stars are

·  Open Cluster – stars not densely packed together

·  Globular cluster – start that are densely packed together

Binary stars

·  When only two stars are gravitationally bound together and orbit a common center of mass.

·  Most binary stars appear to be single stars.

·  More than ½ the stars in the sky are either binary stars or are part of a multiple star system.

Doppler shifts

·  When a star is moving toward the observer, the light emitted by the star shifts toward the blue end of the electromagnetic spectrum.

·  When a star is moving away from the observer, the light emitted by the star shifts toward the red end of the electromagnetic spectrum.

·  This can be used to determine the speed and direction of a stars motion.

Light year

·  The distance light travels in a year. 9.461 x 1012km or 5,878,790,000,000 mi. 3.261 light years = 1 parsec.

Parallax

·  This is the apparent shift in position of a star caused by the motion of the observer. As Earth moves from one side of its orbit to the opposite side, a nearby star seems to be shifting back and forth.

·  The closer the star, the larger the shift.

·  The distance to the star can be determined by measuring the angle of change

Basic Properties of Stars

The basic properties of a star are mass, diameter, luminosity and temperature.

Apparent magnitude

·  This is how bright a star appears to be.

·  The smaller the number, the brighter the star.

Absolute magnitude

·  This is how bright a star is if it were 32.6 light years for Earth.

·  The only disadvantage of this is you can only calculate the absolute magnitude when the actual distance of the star is known.

Temperature

·  O, B, A, F, G, K, M are the letters used to classify the temperatures of stars. O is the hottest and M is the coolest.

·  Temperatures range anywhere from 50,000°K (89,540°F) for O stars to 2,000°K (3,140°F) for M stars.

·  Hotter stars put out more light than cooler stars.

·  Color also indicates a stars temperature. These are the colors in order from hottest to coolest

Composition

·  All stars are made up of nearly the same elements. They are 73% Hydrogen

25% Helium

2% of all other elements

H-R diagram – Hertzsprung-Russell diagram

·  This diagram compares a stars temperature and it’s absolute magnitude.

·  The upper left corner of the diagram is where hot, bright stars are located.

·  The lower right of the diagram is where stars are cooler and dimmer.

·  The middle portion of the H-R diagram is where main sequence stars are located. 90% of all stars including the Sun are main sequence stars.

·  The lower left of the H-R diagram has stars called white dwarfs. These are about the size of Earth. They are small, dim, hot stars but have the mass about as large as the Sun.

·  The upper right of the H-R diagram has stars called red giants. They are very bright, but very cool and extremely large. They are 100 more times the size of the Sun.