The Sun: Part 3and

Measuring the Stars: Part 1

Net result:

4 protons → 4He + 2 neutrinos + energy

Hydrostatic Equilibrium: pressure from fusion reactions balances gravity. Sun is stable.

Mass of end products is less than mass of 4 protons by 0.7%. Mass converted to energy.

600 millions of tons per second fused. Takes billions of years to convert p's to 4He in Sun's core. Process sets lifetime of stars.

Nuclear Fusion of H -> He in the Sun

Fusion as an Energy Source

Can we build fusion reactors on Earth to generate clean (no carbon dioxide) energy?

Maybe.

2H + 3H → 4He + neutron + energy

Trouble is 1) Confinement of the reaction2) safely stopping the neutrons

Methods:1) Magnetic confinement (tokomaks) JET => ITER => DEMO2) Inertial confinement (lasers)

JET tokomak

Sunspots

Roughly Earth-sized

Last ~2 months

Usually in pairs

Follow solar rotation

Sunspots

They are darker because they are cooler (4500 K vs. 5800 K).

Related to loops of the Sun's magnetic field.

radiation from hot gas flowing along magnetic field loop at limb of Sun.

Filament Ejection Movie

Sunspot numbers vary on a 11 year cycle.

0.1% variation from maximum to minimum

Sun's magnetic field changes direction every 11 years. Maximum sunspot activity occurs about halfway between reversals.

Above the photosphere, there is the chromosphere and...

The Corona

Best viewed during eclipses.

T = 106 K

Density = 10-15 g/cm3 only!

We expect X-rays from gas at this temperature.

X-ray brightness varies over 11-year Solar Cycle: coronal activity and sunspot activity go together.

Yohkoh X-ray satellite

The Solar Wind

At top of corona, typical gas speeds are close to escape speed => Sun losing gas in a solar wind.

Wind escapes from "coronal holes", seen in X-ray images.

Wind speed 500 km/sec (takes a few days to reach Earth).

106 tons/s lost. But Sun has lost only 0.1% of its mass from solar wind.

Space Weather

Today’s forecast: solar wind velocity = 331.5 km/s density = 4.3 protons/cm3

Sunspot number: 1days without a sunspot since:

For update see www.spaceweather.com

List of recent and upcoming Near-miss encounters and space related news.

Active Regions

Prominences: Loops of gas ejected from surface. Anchored in sunspot pairs. Last for hours to weeks.

Flares: A more energetic eruption. Lasts for minutes. Less well understood.

Prominences and flares occur most often at maximum of Solar Cycle.

Space weather and solar science● Coronal Mass Ejections: solar

science and ultimately predicting space weather

Solar Probe in 2018

Spectral Classes

Strange lettering scheme is a historical accident.

Spectral Class Surface Temperature Examples

OBAFGKM

30,000 K20,000 K10,000 K7000 K6000 K4000 K3000 K

RigelVega, Sirius

Sun

Betelgeuse

Further subdivision: BO - B9, GO - G9, etc. GO hotter than G9. Sun is a G2.

Classification of Stars Through Spectroscopy

Remember: stellar spectra show black-body radiation and absorption lines.

Pattern of absorption lines depends on temperature (mainly) and chemical composition.

Spectra give most accurate info on these as well as:

density in atmosphere gravity at surface velocity of star towards or from us

Ionized helium. Requires extreme UVphotons. Only hottest stars produce many of these.

Stellar Sizes - Direct Measurement

For a few nearby giant stars we can image them directly using HST or the VLA. Almost all other stars are too far away

Stellar Sizes - Indirect Method

Almost all stars too far away to measure their radii directly. Need indirect method. For blackbodies, use Stefan's Law:

Luminosity (temperature) 4 x (surface area)

Energy radiated per cm2 of area on surface every second T 4

(T = temperature at surface)

Determine luminosity from apparent brightness and distance, determine temperature from spectrum (black-body curve or spectral lines), then find surface area, then find radius (sphere surface area is 4 R2)

And:

Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2)

So:

The Wide Range of Stellar Sizes

How Massive are Stars?

1. Binary Stars. Orbital period depends on masses of two stars and their separation.

2. Theory of stellar structure and evolution. Tells how spectrum and color of star depend on mass.