Stars

Physics 360 - Astrophysics

Brightness

• Different brightness.

• Different color.• How bright are

they really?• What is due to

distance?• What is due to

luminosity?

Spectral Classification

NPOI Observations of MizarA (1Ursa Majoris)

Mizar, 88 light years distant, is the middle star in the handle of the Big Dipper. It was the first binary star system to be imaged with a telescope. Spectroscopic observations show periodic

Doppler shifts in the spectra ofMizarA and B, indicating that they are each binary stars. But they were too close to be directly imaged -until 2 May 1996, when the NPOI produced the first

image ofMizarA. That image was the highest angular resolution image ever made in optical astronomy. Since then, the NPOI has

observedMizarA in 23 different positions over half the binary orbit. These images have been combined here to make a movie of the orbit. As a reference point, one component has been fixed at the map center; in reality, the two stars are of comparable size

and revolve about a common central position.

Orbital Phase: 000o

0.005 arcsec

Mass

Stellar Radii

How big are stars?

• Stars have different sizes.

• If you know:– Distance– Angular size

• Learn real size.

50 mas

Sizes of Stars

Supergiants, Giants and Dwarfs

The Sun, Our Star

• The Sun is an average star.

• From the Sun, we base our understanding of all stars in the Universe.

• No solid surface.

Vital Statistics

• Radius = 100 x Earth (696,000 km)

• Mass = 300,000 x Earth (1.99 x 1030 kg)

• Surface temp = 5,800 K

• Core temp = 15,000,000 K

• Luminosity = 4 x 1026 Watts

• Solar “Day” =– 24.9 Earth days (equator)– 29.8 Earth days (poles)

Interior Properties

• Core = 20 x density of iron

• Surface = 10,000 x less dense than air

• Average density = Jupiter

• Core = 15,000,000 K

• Surface = 5800 K

1. The Core

• Scientific Method:– Observations– Make hypothesis (a model)

• Models make predictions

– Test predictions• Compare results of predictions with observations

– Revise model if necessary.

Testing the Core

• Observe Sun’s:– Mass (how?)– Composition (how?)– Radius

• Use physics to make a model Sun.• Predict:

– Surface temp/density (how do you test?)– Surface Luminosity (how do you test?)– Core temp/density Fusion Rate neutrino rate

(test?)

In The Core

• Density = 20 x density of Iron

• Temperature = 15,000,000 K

• Hydrogen atoms fuse together.

• Create Helium atoms.

Nuclear Fusion• 4H He• The mass of 4 H nuclei (4 protons):

4 x (1.6726 x10-27 kg) = 6.690 x 10-27 kg• The mass of He nuclei: = 6.643 x 10-27 kg• Where does the extra 4.7 x 10-29 kg go?• ENERGY! E = mc

• E = (4.7 x 10-29 kg ) x (3.0 x 108 m/s)2

• E = hc/ = 4.6 x 10-14 m (gamma rays)• So: 4H He + light

2. Helioseismology• Continuous monitoring

of Sun.– Ground based

observatories– One spacecraft (SOHO)

• Surface of the Sun is ‘ringing’

• Sound waves cross the the solar interior and reflect off of the surface (photosphere).

Solar Interior

• Core– Only place with

fusion

• Radiation Zone– Transparent

• Convections Zone– Boiling hot

Convection

• A pot of boiling water:• Hot material rises.• Cooler material sinks.• The energy from the

pot’s hot bottom is physically carried by the convection cells in the water to the surface.

• Same for the Sun.

Solar Cross-Section

• Progressively smaller convection cells carry the energy towards surface.

• See tops of these cells as granules.

The Photosphere

• This is the origin of the 5,800 K thermal radiation we see.

= k/T = k/(5800 K) =480 nm (visible light)

• This is the light we see.

• That’s why we see this as the surface.

Sunspots

• 11-year sunspot cycle.• Center – Umbra: 4500 K• Edge – Penumbra: 5500 K• Photosphere: 5800 K

Magnetic fields and Sunspots

• At kinks, disruption in convection cells.• Sunspots form.

Magnetic fields and Sunspots

• Where magnetic fields “pop out” of Sun, form sunspots.

• Sunspots come in pairs.

Prominences

Hot low density gas = emission lines