Stars

…just kidding

In a nutshell…

• -born from clouds of interstellar gas

• -shine by nuclear fusion

• -shine for millions or billions of years

• -die

• 75% of any star’s mass at birth is Hydrogen, 25% is Helium, no more than 2% is made up of elements heavier than Helium.

• All stars are similar, but they appear different from each other for two reasons:

• -mass

• -we see stars at different ages in their lives

• Were once classified by brightness and location in our sky. This tells us little about its true nature.

• Today, astronomers classify stars according to luminosity and surface temperature.

Luminosity

• -

• the total amount of power it radiates into space, measured in watts

• -the sun’s luminosity is 3.8*10 e26 watts.

• -cannot be measured directly; depends on distance and true luminosity

• Apparent brightness is the amount of light reaching us per unit area

• This obeys the inverse square law!

• (if you double the distance between us and the star, its apparent brightness decreases by a factor of ¼)

• Solar luminosity: compares a star’s luminosity to the sun’s

Measuring apparent brightness• -We do this using a CCD (records how much

energy strikes its light-sensitive surface each second)

• -must calibrate for ground-based telescopes, since the atmosphere of Earth absorbs much of a star’s light

• -when we perceive a star’s brightness, we are only measuring the visible portion of the spectrum

• -total luminosity or total apparent brightness is used to describe the luminosity and apparent brightness IF we could detect photons across the electromagnetic spectrum

Stellar Parallax

• -The small annual shifts in a star’s apparent position caused by the Earth’s motion around the sun.

• the distance to an object with a parallax angle of 1 arcsecond is one parsec (pc)

• 1 pc=3.26 light years=3.09 *10e13 km• d=1/p

The Magnitude System

• -watts per square meter

• -devised by Hipparchus (190-120BC)

• -originally classified stars according to how bright they look to our eyes, since this was the only way of making observations at that time

•

• -brightest stars: first magnitude, second brightest: second magnitude

• -These descriptions are called apparent magnitude, since it compares how different stars appear to us in the sky.

• -Star charts often use different sized dots to represent apparent magnitude.

• Modern times

• -Has been modified: 0 is brighter than 1, -1 is brighter than 0

• -Sirius: apparent magnitude of -1, is the brightest star in the night sky

• Absolute magnitude

• -The apparent magnitude of a star, if it were 10 parsecs away from Earth

• -The brighter the star, the smaller the magnitude!

Stellar Surface Temperature

• -Easier to measure than luminosity, since it does not depend on distance.

• -Determined directly from its spectrum, or color

• -The surface temperature of a star determines the color that it shines

• -Red-yellow-blue (cool to hot)• -Color becomes more apparent when

stars are viewed through a telescope

Spectral Type

• -Emission and absorption lines provide a more accurate way to determine its surface temperature.

• -Ionized elements (hot), molecules (cool)

• -Astronomers classify stars according to surface temperature by assigning a spectral type from the types of spectral lines present in a star’s spectrum.

• O=Blue

• B=Blue-white

• A=White

• F=Yellow-white

• G=Yellow

• K=Orange

• M=Red

Binary Systems

• Visual binary: A binary star system which we can observe with the eye.

• These stars orbit each other, and appear to change positions. These are

• Rare!• -Sometimes, we can observe a shift in position, but not

see the companion, because it is too dim to be seen. This is called an eclipsing binary.

• -If a binary system is neither visual nor eclipsing, the only way we can determine that it has a partner is from Doppler shifts in its spectral lines. This is called a spectroscopic binary.

The Hertzprung-Russel diagram

• -Used to plot the surface temperature vs. the luminosity of stars, and classify them using these characteristics.

•• Patterns in the H-R diagram• -Most stars fall along the main sequence• -stars in the upper right are called supergiants,

they are very large and very bright• -just below the supergiants are the giants, which

are smaller in radius and lower in luminosity• -the stars in the lower left are small in radius, and

appear whit in color because of their high temperature, these are white dwarfs.

• stars are classified by their spectral type and luminosity class that describes the region of the diagram in which the star falls.

• Class represents• I supergiants• III giants• V main sequence• *II and IV intermediate • -our sun’s classification is G2 V

The Main Sequence

• These stars are fusing hydrogen into helium in their cores.

• Most stars call along this line, since most of their lives is spent fusing H into He.

• Stellar masses increase upward along the main sequence.

• More stars fall on the lower end of the main sequence than on the upper end, which tells us that low-mass stars are more common than high-mass stars

• The sun is an average main-sequence star.• A star’s main sequence lifetime is determined by

its supply of hydrogen.• More massive stars live shorter lives because

they use up their hydrogen at a much faster rate than less massive stars.

• The main sequence lifetime for our sun is 10 billion years. It is currently middle-aged.

Giants and Supergiants

• Nearing the ends of their lives because they have exhausted their core H.

• Stars grow more luminous when they begin to run out of fuel.

• These stars can be seen even when they aren’t close to us.

• Often identifiable by their reddish color.

• These are rarer than main sequence stars because we catch most stars in the act of hydrogen burning, and relatively few in their later stages of life.

• These eventually run out of fuel completely., and become white dwarfs, which are roughly the size of Earth.

Pulsating Variables

• Have atmospheres that alternately expand and contract, causing the star to rise and fall in luminosity.

• Most inhabit the instability strip on the H-R diagram that lies between the main sequence and the red giants.

Star Clusters

• All stars are born from clouds of gas, and most form in groups.

• Open and globular clusters

Open• Always found in the disk of the galaxy

• Can contain up to several thousand stars and span 30 light years.

• Pleiades: in the constellation Taurus, called the Seven Sisters.

Globular

• Found in both the disk and the halo of our galaxy

• Can contain more than a million stars, concentrated in a ball, 60-150 light years across.

• These are both useful to astronomers because all of the stars are at the same distance from Earth, and all of the stars formed at relatively the same time.

Cluster ages from Main Sequence Turnoff

• The precise point at which a cluster’s main sequence diverges from the standard main sequence

• Age of cluster = lifetime of stars at main sequence turnoff point

• This is the most powerful tool for evaluating the ages of star clusters.

SUMMARY

• All stars are made primarily of hydrogen and helium.

• Much of what we know about stars comes from studying the patterns that develop when we plot stellar surface temperature and luminosity on the H-R diagram.

• Stars spend most of their lives on the main sequence, fusing hydrogen into helium.

• Much of what we know about the universe comes from the study of star clusters.