Astronomy 1020Spring_2015

Day-33Stellar Astronomy

Course Announcements• 1 Dark night observing session left:

• Thurs. Apr. 16• Alternative exercise is posted.

• Reports are due Wed. Apr. 22• Fri. 17th: 1-5pm in MUC – Research & Creative

Forum

• LABS:• This Week: Eclipsing Binaries• Next Week: Hubble Red Shift• “On your own”: Galaxy Zoo Classification

• Due: Wed. 29th at class time (NO late labs accepted)

Evolutionary Tracks

Protostars get less luminous (for lower masses), smaller in radius, and hotter.

The star moves on the Hayashi track and arrives on the main sequence.

The more massive the protostar, the more rapidly it evolves

Concept Quiz Evolutionary Tracks

Once fusion begins, a star moves to the left on the H-R diagram. Its luminosity does not change, but its temperature rises. The star is:

A. Expanding.

B. Contracting.

C. Staying at the same radius.

D. Can’t tell from the information given.

Many or all protostars have material leaving in a bipolar outflow of jets.

Infalling and outflowing gas can be very complex.

Bipolar Outflows Powerful jets can

collide with the interstellar medium to make Herbig-Haro (HH) objects.

These can eject muchof the mass that would otherwise land on the star.

Star formation can make star clusters.

These are gravitationally bound groups of stars.

Clusters are good laboratories for testing our ideas of star formation and evolution.

Star formation may take millions of years.

Some stars are more massive; others less so.

Higher-mass stars take less time forming and evolving.

Star Clusters

All stars are: - same age- same composition-same distance

Only difference:-mass

New investigative methods can reveal misunderstandings.

Astronomers did not realize the presence and effect of gas and dust on starlight until spectroscopy was developed and applied.

PROCESS OF SCIENCE

A brown dwarf is not a star, nor a planet, but is in between.

Classified as L, T, or Y (cooler than M stars). Glow in the infrared due to internal heat from

gravitational contraction. Over 1,000 have been found since the mid-

1990s.

CONNECTIONS 15.1CONNECTIONS 15.1

Stars are constantly radiating energy. The energy available from fusion is very

large, but finite. Eventually, the fusion sources change, then

run out.

The star’s luminosity, size, or temperature will change.

A star’s life depends on mass and composition.

Stars of different masses evolve differently.

The rates and types of fusion depend on the star’s mass.

Generally, stars with M < 3 M share many characteristics: low-mass stars.

Intermediate-mass stars: 3 M < M < 8 M

High-mass stars: M > 8 M

Higher temperature and pressure means faster nuclear fusion.

We can figure out main-sequence lifetimes:lifetime = (energy available) / (rate used).

More mass = more fuel available.

Rate energy used = luminosity.

More massive stars have much higher luminosity.

They use their fuel up more quickly and leave the MS faster.

Estimates can be made of star lifetimes, based on mass.

The mass-luminosity relationship: The lifetime of a star depends on the amount of

fuel (M) and how quickly it is used (L). Can use this to compare other stars to the Sun:

MATH TOOLS 16.1MATH TOOLS 16.1

Main-sequence stars fuse hydrogen to helium in their cores.

Eventually, much of the core H is converted to He.

A core of He ash is built up (does not fuse at this point).

Helium Core Is Degenerate H fusion only takes place in a shell around the 100

percent He core: hydrogen shell burning. If H fusion is not happening in the core, the star is

no longer main sequence. Since the He is not fusing, gravity begins to win

over the pressure, crushing the He. The core becomes more dense, and becomes

electron-degenerate. This means pressure is not from moving atoms,

but from a quantum mechanical effect: There’s a limit to how tightly electrons can be packed together.