Other Suns

Distant Stars

What is a Star?

• Stars begin as cold molecular hydrogen that its own gravity causes to collapse.

• As it collapses, it fragments and these fragments form stars because of nuclear fusion at the core.

• Gravity pushes inward, the energy from the core pushes outward. This balances the star.

Finding distance to stars

• 1. Absolute Magnitude: A scale which uses a specific distance to compare the brightness of stars. (10 parsecs, or 32.6 Light years, almost our sun distance.) Means the brightness a star would have at that distance. (Similar to Luminosity)

• With the scale THE LOWER THE NUMBER THE BRIGHTER THE STAR.

How Powerful are the Stars?

• Luminosity: Amount of energy a star gives off in a given amount of time.

• Analogy:Think of headlights: Car headlights are basically the same. But the closer they are the brighter they are. So the same headlights can show different brightness depending on the distance they are away.

So Stars can have the same brightness but we see them differently because of distance.

Here is how it works

• 1. Calculate how bright a star would appear at standard distance of 32.6 lt-yrs. So all stars are compared at that distance. This is called absolute magnitude.

• For a star at this distance is has an absolute magnitude and apparent magnitude that are the same.

Explanation step

• 2. Consider a star that is farther away from us than the standard distance. To get absolute magnitude you bring it closer. That means its magnitude at standard distance is brighter than apparent magnitude (brightness at actual distance).

1 Continued: Apparent Magnitude

Apparent Magnitude is the brightness of a star at its actual distance.

Apparent Magnitude is calculated using a telescope.

Compare Absolute Magnitude to Apparent Magnitude gives the distance to the star.

2. Trigonometric Parallax

parallax = “p” is defined as half of the angular shift produced over a 6 month baseline (2 au) which is the diameter of the earth’s orbit.

Parallax explained.

• Example: The distance of a star whose parallax is 1 is 1 parsec. 1 pc is 3.26 lt-yrs.

• It turns out then that the star’s distance in parasecs is the inverse of its parallax in seconds of arc:

• d = 1/p

Examples

• A star whose measured parallax is .5 arc seconds has a distance of 1/.5 or about 6.5 lt-yrs.

• The closest star Proxima Centauri has a parallax of .77 arcseconds so how many light years away is it?

• 1/.77 = 1.3 pc 1.3 pc = ? lt yrs? • 4.2 lt-yrs

3. b = L/(4pd2)

• A star’s brightness (b) depends on its distance because of how the light spreads out uniformly in all directions.

The larger the distance

the greater surface area

of the sphere that

the light spreads out

over, so the smaller the

amount of

light that will pass

through a section of the

sphere.

Inverse Square Law

• The energy we receive from a star (luminosity) decreases with

• the square of

• the distance

• the star is

• away.

Summary

• 3 ways to find distance to stars. • 1. use telescope, find apparent

magnitude, calculate absolute magnitude and compare them.

• 2. use relationship between parallax and distance d = 1/p

• 3. use luminosity and brightness• b = L/(4pd2) where d is distance to

star.

Proper Motion

• Proper Motion: Stars motion in sky relative to OTHER stars, not Earth.

• Units are typically arc/century.

• Easier to measure stars motion toward or away from us rather than side to side.

Doppler Effect

• Reminder of Doppler Effect:

• change in

• the spectrum

• because

• the source

• is moving.

Red Shift/Blue Shift

• Blue Shift means the star is moving toward us because the wavelengths get shorter.

• Red Shift means the star is moving away from us because the wavelengths get longer.

• Song?? http://www.astrocappella.com/doppler.shtml

eye eye

How Fast do stars go?

change in wavelength = speed of emitterrest wavelength speed of light

speed of emitter is speed of source either toward or away from us

rest wavelength is emitted by a source of light that is not moving relative to the observer.speed of light is 3E8 m/s.

Classification of Stars

• Main Sequence: On a • Temperature -Luminosity diagram stars at the main sequence areburninghydrogen into

helium.

OBAFGKM

Spectral type in order of hottest to coolest.

More recently: Cooler objects like brown dwarfs and failed stars.

Absolute Magnitude vs. Spectral Type

• Same graph results plotting Absolute Magnitude (equivalent to luminosity) vs. Spectral type (derived because of temperature)

BinariesBinary star systems have enabledastronomers to deduce characteristicsand make up of the stars as they influence one another.

Some orbit black holes: One such system consists of a blue supergiantstar and a suspected black hole whichhas 14.8 times the mass of the Sun.

Some binaries are invisible: The second star might be a brown dwarf or neutronstar, only making itself known through its gravitational influence on its partner.

They cause Supernovae: Binaries transfer mass between them. When one star takes on so much mass it can no longer support itself and it explodes.

4 out of 5 stars are multiples.

Variable Stars

• Stars that change brightness because of an eclipsing spectroscopic binary.

• A very famous one is Algol which can be seen in the North American sky.

• http://csep10.phys.utk.edu/astr162/lect/binaries/algol.html

Some Star Stories

• http://www.astrocappella.com/background/wolf_background.shtml

• http://www.astrocappella.com/activities/land_safely.html

• Binaries: http://www.space.com/22509-binary-stars.html