Test 3 (PHYS 105)

What are population I and II stars?

Population 1: young, metal-rich star probably found in an open cluster. Formed from . . . . Population II: older, metal-poor star found in a globular cluster

What is the horizontal branch?

Helium-burning stars are found on a horizontal branch on the H-R diagram.

What kinds of objects become protostars?

Raw material?

What is the idea of the Jeans instability? What roles do temperature and density play?

Specifies that if a region of gas reaches a certain density, at a low enough temperature then gravity will contract it enough to form a new star.

What are the main constituents of the interstellar medium?

The gas between the stars is called the interstellar medium (10% of the Milky Way by mass). Stars form in dark clouds of dusty gas in interstellar space.

What is accretion?

The gravitational binding of matter onto a central object.

What is the Oppenheimer-Volkoff Limit?

If a compact core exceeds 3 M⊙(the Oppenheimer-Volkoff limit) neutron star degeneracy pressure fails and the star completely collapses to an infinitely dense point. No known force can stop the collapse, thus a black hole.

What is the Chandrasekhar limit?

If a core exceeds 1.4 solar masses, electron degeneracy pressure fails and the core will collapse. This sets an upper limit to the mass of a white dwarf.

How are black holes identified?

Isolated black holes are nearly impossible to discover. Most black hole candidates are discovered through their affect on their surroundings. Accreting galactic black holes in binary systems are identified through mass estimates and high-energy emissions.

How do we detect the IM?

We can determine the composition of interstellar gas from its absorption lines in the spectra of stars. Long-wavelength infrared light passes through a cloud more easily than visible light. Observations of infrared light reveal stars on the other side of the cloud.

What is the triple alpha process?

When the core reaches 100 million K (108 K), helium fusion begins through the triple-alpha process: 4He + 4He + 4He

What is the sequence of shell and core burnings in a low-mass star? Which are associated with each stage?

- triple -alpha creates C/O cores - temperature too low to fuse C/O (requires > 600 million K) - star contracts and begins helium shell fusion Helium-shell burning expands the star up through the asympotic giant branch (AGB stars) as red supergiants These stars are highly variable (Mira variables) and lose mass from a strong stellar wind (> 3/4 mass lost in all giant stages) Convection becomes very important post-main sequence. Convection at various stages dredges up heavier elements from the center, dispersing them back into space. After core helium fusion stops, helium fuses into carbon in a shell around the carbon core, and hydrogen fuses to helium in a shell around the helium layer. This double shell-burning stage never reaches equilibrium—fusion rate periodically spikes upward in a series of thermal pulses. With each spike, convection dredges carbon up from core and transports it to surface.

What is the general nature of a black hole?

A black hole is a massive object whose radius is so small that the escape velocity exceeds the speed of light. You can orbit a black hole like any other object of the same mass—black holes don't suck! - Near the event horizon, time slows down and tidal forces are very strong

How do high-mass stars evolve?

A high-mass star raises temperatures enough to fuse increasingly heavy elements

What is a nova lightcurve like?

A nova lightcurve appears in spikes.

What defines a pre-main sequence star?

A pre-main-sequence star is a star in the stage when it has not yet reached the main sequence. Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope of interstellar dust and gas.

How is mass loss related to star formation?

A prominent part of the process of stellar formation and evolution is mass-loss - the ejection of outer stellar envelopes into space. Mass loss occurs at many stages.

What is a pulsar?

A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis.

What is a red giant star? About what temp/luminosity/size does it have?

A red giant is a luminous giant star of low or intermediate mass (roughly 0.3-8 solar masses (M☉)) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, from 5,000 K and lower.

What defines a protostar?

An accreting core destined to be a star - not yet fusing elements. It is made up of gas and dust. First few thousand years - contraction converts gravitational energy to thermal energy, heating up the protostar. Protostar becomes more luminous because of the increased in temperature to >1000 Kelvin. Can be 100x brighter than the Sun.

What causes the red giant phase?

As the core contracts, H begins fusing to He in a shell around the core. Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting. The thermostat of a low-mass red giant is broken because degeneracy pressure supports the core

How does a protostar change with time? About how big is it originally?

First few thousand years - contraction converts gravitational energy to thermal energy, heating up the protostar. Protostar becomes more luminous because of the increased in temperature to >1000 Kelvin. Can be 100x brighter than the Sun. Size of core shrinks to 10-20 times that of the Sun during this stage. It is originally around ~100 M⊙ spread over more than 100 A.U.

What are some effects of the near-horizon environment?

General relativity describes some interesting effects near a black hole: •Clocks run slower (time slows down) •Light is shifted to longer wavelengths (gravitational redshift) •Tidal gravitational force changes quickly with distance Near the event horizon, time slows down and tidal forces are very strong.

What is ZAMS?

HR diagram position where stars stabilize.

What defines a main sequence star?

Has reached an equilibrium stage of hydrogen fusion.

Would an infalling observer necessarily note the passage through an event horizon?

No, they would not.

What causes a nova?

Nova - burst of hydrogen fusion raising the luminosity and causing an energetic outflow of gas (can repeat)

How has the sun changed during its main sequence life? What causes these changes?

Our Sun expands for 700 million years, getting 100x larger, temp dropping to about 3500 K

Planetary Nebula? How long are they visible for? Why?

The mass loss of AGB stars is driven by the expansion of the star from repeated helium flashes. The gas propelled outward cools and condenses, appearing as an expanding cloud of dust and gas known as a planetary nebula. Double shell burning ends with a pulse that ejects the H and He into space as a planetary nebula. The core left behind becomes a white dwarf. 50,000 years

How does the main sequence lifetime depend on star mass?

The mass of a main-sequence star determines its core pressure and temperature. Stars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter-lived. Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes.

What are the limits to main sequence star masses? What established these?

The top limit is iffy. Degeneracy pressure.

What is the main challenge in understanding stellar evolution?

Time.

What is dust?

Tiny solid particles of interstellar dust that block our view of stars on the other side of a cloud.

How do radio and accreting pulsars differ?

radio pulsar: rapidly spinning (P < 1 sec) neutron star seen to pulsate as the rotation sweeps a radiation beam through our line of sight from a magnetic field Accreting pulsar: pulsar seen because matter accreted from the companion flows down onto the spinning neutron star (P > 1 sec)