What is a Star?What is a Star?

Wednesday, October 8Wednesday, October 8Next Planetarium Shows: Tonight 7 pm, Thurs 7 pmNext Planetarium Shows: Tonight 7 pm, Thurs 7 pm

Light source moving towardtoward you: wavelength is shorter (blueshift).

Light source moving away away from you: wavelength is longer (redshift).

Doppler shift review:

Size Size of Doppler shift is proportional to radial velocity:

cV

0

Δλ = observed wavelength shift = λ-λ0

λ0 = wavelength if source isn’t moving

V = radial velocity of moving source

c = speed of light = 300,000 km/sec

Example:

Hydrogen absorbs light with λ0 = 656.3 nm.

You observe a star with a hydrogen absorption line at λ = 656.2 nm.

What is a star?Examples of stars:

Sun Betelgeuse

What is a star?

A large, hot, luminous ball of gas.

“Why do stars shine?”

Stars are dense (Sun is 40% denser than liquid water).

Stars are opaque (you can’t see to the Sun’s center).

Stars are hot.

What happens when a dense, opaque

object becomes hot?

It emits light.

What do I mean by ““HOT”HOT”?

90°F 9980°F212°F

At the submicroscopic level: atoms in a gas

Object is hothot when the atoms of which it’s made are in rapid random motion.

TemperatureTemperature: measure of typical

speed of the atoms.

Random motions stop at absolute zeroabsolute zero temperature.

Kelvin = Celsius + 273Water boils: 373 Kelvin (K)

Water freezes: 273 K

Absolute zero: 0 K

Room temperature: ~300 K

Surface of Sun: ~5800 K

Different elements respond in different ways to changes in temperature.

At high density & low temperature, hydrogen is a gas of molecules.

Molecular hydrogen = H2 = two H atoms bonded together

(This assumes there’s no oxygen for the hydrogen

to bond with.)

At low density & low temperature, hydrogen is a gas of atoms.

Much of the interstellar gas in our Galaxy is

atomic hydrogen.

density ≈ 10 atoms/cm3 T ≈ 100 K

At high density & high temperature, hydrogen is an ionizedionized gas.

Much of the Sun’s interior is ionized

hydrogen.

Sun’s center: density ≈ 150 tons/m3

T ≈ 15 million K

A blackbody is an object that absorbs allall the light that hits it.

Heat a blackbody: it emits light of all wavelengths (a continuous continuous spectrum).

Wavelength at which spectrum peaks depends only on temperatureonly on temperature.

A star is an approximate blackbodyblackbody..

Blackbody spectra

21,100°F

10,300°F

4900°F

Wavelength of peak emission for a blackbody is inverselyinversely related to

temperature.

Tnm 000,900,2

peak

emission maximum ofh wavelengtpeak

(Kelvin) re temperatuT

Examples:You:

Sun’s surface:

310

nm 000,900,2peak

5800

nm 000,900,2peak

Kelvin 310 C 37 F 6.98 T

Kelvin 5800T

9400 nm (Infrared)

500 nm (Visible)

2 am tomorrow, looking east

Another example: taking the temperature of a star!

Betelgeuse is redred.

Rigel is blueblue.

(Hard to see colors with the naked eye – binoculars help!)

Betelgeuse:

Rigel:

1000

K 000,900,2T

200

K 000,900,2T

nm 1000peak

nm 200peak

2900 K (4800°F)

14,500 K (26,000°F)

The Sun’s actual spectrum:

Close to a blackbody, but not perfect.

Perfect blackbody

Inte

nsity

Friday’s Lecture:

Reading: Chapter 3

What is a Galaxy?