Astronomy Exam 3 Buta

What is a star?

Big spheres of mostly charged particles. 98% hydrogen and helium and 2% of heavier elements.

What is a white dwarf supernova?

Detonation of a white dwarf in a titanic explosion.

How do stars compare to planets?

Much more massive than any planet.

Why are sunspots dark?

Darkness mean less light or no light (cooler regions of Sun).

What do sunspots tell us about solar rotation?

Tells us the sun rotates, not as a solid object, 25 days at equator, 35 days at poles.

What is the evidence that convection operates just below the surface of the sun?

Granules - Hot bubbles of heat rising gas carrying heat from inside the sun to its surface, where it is released into space.

How does the appearance of the Sun change with wavelength?

The Sun emits all wavelengths of light, but it looks different at different wavelengths

X-rays Ultraviolet Visible Infrared Radio

Increasing ---->

Most of the sunlight comes out at visible wavelengths.

What kind of spectrum does the Sun have?

When sunlight is dispersed, a rainbow spectrum of colors is seen interrupted by hundreds of dark absorption lines. These lines originate in the atmosphere of the Sun and occur at wavelengths depending on the composition of the gases.

What powers the Sun's luminosity?

Nuclear Fusion

Know the structural layout of the Sun

Core

radiation zone

convection zone

photosphere

chromosphere

corona

What is the sunspot cycle?

Cyclic variation in the number of sunspots. The period of variations is about 11 years.

What phenomena are enhanced at sunspot

maximum?

Solar flares, extended corona, magnetic loop prominence, enhanced solar wind, an excessive number of sunspots.

What does it mean to say that the Sun is in gravitational equilibrium?

The sun is in a condition where the mass above every point inside is exactly balanced by the outward thermal gas pressure at that point.

What is "outward thermal gas pressure"?

The gas pressure increases with increasing depth.

What are the photosphere, chromosphere, and corona?

Photosphere - The photosphere is the visible surface of the Sun. The temperature at this surface, which is purely gaseous, is 5800K.

Chromosphere - The chromosphere is the colorful lower layer of the solar atmosphere, seen most easily during a total solar eclipse.

corona - The corona is the hot outer atmosphere of the Sun. It is also best seen during a total solar eclipse.

How do we determine the masses of stars?

Parallax plus existence of binary stars

How do we determine the luminosities of stars?

Parallax plus apparent brightness

How do we determine the chemical compositions of stars?

spectroscopy

How do we determine the surface temperatures of stars?

color and spectral classification

Which of the above properties require knowledge of the distance, and which do not?

You need to know the distance to figure out the luminosity, radius, and the age of single stars not in a cluster.

What is the chemical composition of a typical star near the Sun?

98% hydrogen and helium and 2% heavier elements

What is spectral classification?

Spectral classification uses the relative strengths of absorption lines to estimate surface temperatures.

What is the spectral sequence OBAFGKM?

Sequence of decreasing surface temperatures.

What does a type like "K7" or "B1" mean?

Each spectral class is subdivided to get finer determinations of surface temperature. 0 is hottest, 9 is coolest (of the class)

How are distances to nearby stars determined?

By trigonometric parallax: The apparent shift in position of a nearby star relative to more distant background stars.

When large numbers of stars are analyzed, what ranges do we find for their surface temperatures, luminosities, masses, and radii?

A wide range

What is an H-R diagram?

A graph of luminosity v. surface temperature (amount of lights v. color of the stars)

What are main sequence stars?

Stars that lie along a band of the upper left to the lower right

What are red giants?

Lies above the lower main sequence. contains large, cool stars.

What are white dwarfs?

Below the line of the main sequence stars. Dim, hot stars. Earth sized.

What are super giants?

A band of stars that extend to the right of the upper main sequence. Really large stars

What do I, III, and V mean in a spectral

classification?

I: a supergiant

III: a giant

V: a dwarf (applied to all main sequence stars)

What are the spectral type and luminosity class of theSun?

G2 V star

What is the significance of the main sequence? How does mass correlate with position on the main sequence?

- 90% of stars fall into this category

- Only these stars have the mass-luminosity correlation

-Only these are powered by core hydrogen fusion.

How does main sequence lifetime depend on mass?

The more massive a star, the higher its luminosity, which means it will be more towards the top and have a shorter lifespan.

What is meant by stellar evolution? What does the theory tell us?

Theory of stellar evolution tells us that:

-Stars are born as main sequence stars

- dense cold pocket in interstellar cloud

- gravitational collapse

- cons. energy > rising central temp.

- onset of core hydrogen fusion into helium

- gravitational equilibrium.

Red Giants, supergiants, and white dwarfs are evolved stars

- no star has an unlimited supply of core hydrogen

- when the fuel runs out, they evolve.

What is gravitational equilibrium?

Exact balance between mass and gravity.

How and where do stars form?

Stars form in cold, dense pockets of gas and dust in an interstellar cloud.

What is a protostar?

A protostar is a continuously shrinking mass that heats up as it contracts.

-As it shrinks, its core temp rises for fusion to produce extra energy.

-Onset of nuclear fusion leads to gravitational equilibrium for a time and births a stable star.

What does a star-forming region look like?

A star forming region looks like a giant cluster of several tens to several hundred stars.

How do we know that pink nebulae are stellar nurseries?

When a cluster is super young, the older stars cause the unused surrounding hydrogen gas to glow pink.

Only spectral O stars can do this, but they have a short life span.

What is the main sequence phase?

The longest phase of stellar evolution

What powers the star's luminosity during this phase?

hydrogen fusion in the core

When a main sequence star runs out of fuel, what happens?

The core contracts to raise the temperature to burn what the byproducts of the previous phase was fuel.

What is meant by life track?

A life track is a simulation by a computer program to predict how stars evolve.

What characterizes the life tracks of low and high mass stars?

Life tracks are strongly dependent on mass of a star.

What are the rules of interstellar evolution?

-High mass stars evolve faster in all phases.

-In order to power a star, fusion must release energy.

-When a star runs out of fuel, the core must contract to raise the temp. to try and turn the remnants of the previous stage into new fuel.

What are the phases of stellar evolution of a sun like star?

1. Main sequence star

2. Super Giant

3. Red Giant

4. Helium-burning star

5.Double shell-burning star

6. Planetary nebula

7. White Dwarf

-After MS phase, star becomes a red giant

-then a core-helium fusing yellow giant

-then becomes an even bigger red giant a second time

-then ejects ints envelope gases and leaves behind a white dwarf.

How do these phases map out onto an H-R diagram?

Like a zigzag

What powers a 1 solar mass star during: (A) 1st red giant (B) yellow giant phase (C) 2nd red giant phase?

A - Shell H fusion

B - Core He Fusion and Shell H fusion

C - Core He and H fusion

What is a planetary nebula?

-Instabilities in He-fusing shell causes envelope gases to separate from the inert carbon-oxygen core

-As envelope gases expand, the very hot core becomes exposed to space. It's UV light photoionizes the envelope gases, producing the glowing nebula.

What is the nature of the dim central stars in these nebulae?

-Carbon-oxygen cores of 2nd stage red giants

-As it cools, it becomes a white dwarf.

What balances a white dwarfs mass against gravity?

Electron degeneracy pressure is what balances it out.

What is degeneracy pressure?

High energy electrons that produce outward pressure that oppose the white dwarf's gravitational pressure and keeps it in an equilibrium.

How do high mass stars evolve compared to low mass stars?

-After MS phase, high mass stars evolve to right in H-R diagram.

-Become red supergiant

-explosively eject their enveloped ad leave behind a neutron star.

What is a supernova?

A supernova is a violent core collapse and the violent ejection of the gaseous envelope

How does core collapse produce a neutron star?

It transforms from a earth ball of iron nuclei to a city sized neutron star.

Why is iron a death sentence for a massive star?

Iron is the one chemical element where neither fusion nor fission can release energy. Iron fusion cannot power a star.

How do supernovae enrich interstellar gases?

The gases from supernovae return to space enriched with heavy elements and mix with other materials.

How does the correlation between rate of stellar evolution and stellar mass tell us the age of a star?

-High mass stars evolve faster than low mass in all phases of stellar evolution

-More low mass than low mass

What is the main sequence turn off?

Where the main sequence stars stop existing the line in the HR diagram bends.

What are open clusters?

Few hundred to few thousand stars

-bright

-wide range of ages

-about 2% are heavy elements (metal-rich)

What are globular clusters?

About a million star

-brightest stars are red giants

-all old

-metal poor

What do the H-R diagrams of young and old star clusters look like?

Old star clusters have branches in their HR diagrams

What clusters actually show core helium-fusing (horizontal branch) stars in their HR diagrams?

Globular star clusters

What have HR diagrams of open and globular star clusters told us?

The globular clusters are defiant in heavy chemical elements

What is a white dwarf?

The exposed cores of low mass, sun-like stars that have shed their outer layers. Composed of carbon and oxygen and smaller ones mainly of helium.

How big are white dwarfs?

Very small and dim, their mass is comparable to earth.

What powers the luminosity of a white dwarf?

Stored heat

What is the white dwarf limit?

White dwarf cannot be more massive than 1.4Msun.

How big are neutron stars?

Masses: 1.5 - 3 Msun at most.

What are pulsars? How do they connect to neutron stars?

Pulsars are neutron stars by a different name.

What is the light house model?

These particles beam light from the magnetic poles. If the magnetic poles are not aligned with the rotation axis, then as the neutron star rotates it sweeps its beams around the sky, like a lighthouse.

What major discovery linked pulsars to supernova?

Pulsars come from supernovae explosions

What is a black hole?

A dead super high mass star.

How do black holes fit into the grand scheme of stellar evolution?

They are the end of their high mass stars.

What kinds of stars can become black holes?

The most massive stars (spectral class O stars).

What is the singularity of a black hole?

A point of zero radius and infinite density.

What is the event horizon of a black hole?

An imaginary spherical surface where the escape velocity equals the speed of light.

It is the point of no return.

If the sun suddenly shrank into a black hole tomorrow, what would happen to earth?

We would keep orbiting as we are now but have no heat and light. Gravity would be different only near the event horizon.

What conditions can lead to mass transfer in a binary star system?

2 main sequence stars in a close binary system, and one star becomes a red giant and fills the Roche lobe and dumps mass into its companion star.

What are Roche Lobes and what is the significance of the connecting point between them?

Roche lobes are the gravitational domain of each stars. The lagragian point is where these lobes connect and this is where mass transfer occurs.

What is an accretion disk, and why does one form?

You get a disk because everything is moving and all the mass is collecting around a moving star.

What are bipolar jets?

Material falls on the neutron star so fast that some gas is beamed out of the poles of the accretion disk. These beams are called jets.

What accounts for the extreme phenomena which occur when mass is dumped on a white dwarf?

A type 1A supernova.

What is a nova?

Nuclear explosion on the surface of a white dwarf.

What kinds of phenomena occur when a neutron star is in a close binary and receives mass from its companion?

You get an accretion disk just as with a white dwarf, but the disk is so hot that the X-rays are emitted. If you want to find accretion neutron stars, first look for x-ray sources.

What happens when mass is dumped onto a black hole?

-First settles into an accretion disk

-Gets heated to emit x-rays

-Bipolar jets may be present if the rate at which mass is falling into the hole is too high

How can you be sure that an X-ray source involves a black hole and not a neutron star?

By estimating the mass of the dead star.