Exam 3 to End Flashcards
Going up the main sequence (red to blue stars) mass, radius and luminosity all…
Increase
Which stars are more luminous: high-mass or low-mass?
High-mass stars
High-mass stars burn their fuel supply much _____ than low-mass stars.
Faster
Star lifetimes:
Red dwarf (0.1 M@) —->
Sun (1 M@) —->
Blue supergiant (10 M@) —->
Star lifetimes:
>10^12 yr
10^10 yr
2 x 10^7 yr
Which stars move through all phases faster: high-mass or low-mass?
High-mass stars
Where do stars form?
In cold, dense interstellar clouds
What is a dark nebula?
A dense cloud of gas and dust, which absorbs visible light and obscures stars behind it.
What is cloud collapse and what causes it?
When a cloud goes out of hydrostatic equilibrium. This is caused by gravity, meaning the cloud is compressed by a shockwave.
What are possible triggers of a cloud collapse?
A nearby supernova, a cloud passing through a galaxy’s spiral arm, or cloud collision
True of false: As gravitational contraction of a cloud continues, gas is compressed and heated.
True
Protostars are surrounded by what (which is also where they form)?
Dense gas and dust clouds
How are protostars best observed?
In infrared
Which is less absorbed by dust: Visible light or infrared?
Infrared
When does H-fusion begin in the core of a protostar?
When its central temperature reaches 10^7 K
Which stage begins a star’s main sequence?
A protostar
What does an H-R diagram show?
A star’s evolutionary track as it’s luminosity and temperature change.
What kind of luminosity and temperature does a protostar have?
High luminosity and low temperature
What is a reflection nebulae?
A bluish haze seen around hot, young stars due to interstellar dust.
A reflection nebulae’s dust reflects blue or red light more effectively?
Blue light (the red light appears redder than its true color)
What is interstellar reddening?
When dust clouds between a star and its observer scatter more blue light than red light out of its beam, which makes the star appear redder than its true color.
Why does Earth’s sky appear blue?
Because air molecules reflect more blue light than red light.
Why does the sun look red near the horizon?
Because more blue light than red light has been removed and the light travels through more atmosphere near the horizon.
What are lower end main sequence limits?
Below 0.08 M@ when the central temperature is too low for fusion
What are upper end main sequence limits?
Above about 100 M@ when the radiation pressure blows stars apart
What are the evolutionary stages of a 1 M@ Star?
- Main sequence
- Red giant
- Helium flash
- Horizontal branch
- Second red giant
- Planetary nebula
- White dwarf
Nuclear fusion in a main sequence star’s core converts…
Hydrogen to Helium
When does a star leave the main sequence?
When the hydrogen is used up in its core
How long will the sun spend on its main sequence?
10^10 years
True or false: More massive stars burn through Hydrogen slower?
False, they burn through it faster.
When H fusion ends in a star’s core, does it continue somewhere else during the red giant phase?
Yes, in the shell around the core.
What causes a star’s core to contract and heat during the red giant phase?
Gravity
During the red giant phase, the star’s outer region…
Expands and cools due to increased pressure
What are some red giant properties?
Big, bright, and cool
R = 100 R@ = 0.5 AU
L = 1000 L@
T = 3500 K
What is a helium flash?
When the central temperature of a red giant is 10^8 K and Helium fusion begins in its core.
What is the triple-a process?
3 He —-> C + energy
He = helium nucleus = a-particle C = carbon nucleus
What fuses in the horizontal branch phase of a star?
Helium fuses to carbon in its core, some Helium fuses with carbon to form oxygen.
What does the star’s core do in the horizontal branch phase?
It expands, cools and becomes bluer.
Which luminosity is higher: the sun or a horizontal branch star?
A horizontal branch star’s luminosity
True or false: Horizontal branch stars form a horizontal grouping in the H-R diagram.
True
What stage is a star in when there is no Helium left in the core and its core is now made up of carbon and oxygen?
Second red giant phase
In the second red giant phase, what does the core do and what does the envelope do?
The core contracts and heats, while the envelope expands and cools.
Towards the end of the second red giant phase, the star becomes…
a red supergiant
During the planetary nebula phase, what does the red supergiant’s envelope do?
It pulsates unstably, eventually becoming ejected and exposing the star’s core.
What is the structure of the star during its planetary nebula phase?
It has an expanding spherical gas shell around the small, hot star and the shell appears as a bright ring.
The structure of other planetary nebulae’s ejected gas can be nonspherical due to…
A star orbiting a binary companion, or a disk around the star producing an hourglass shape.
Hot, dense core becomes a white dwarf =
collapsed star about the size of the Earth
What is the density of a white dwarf?
10^9 kg/m^3 = 10^6 x density of water
What is the structure of a white dwarf?
Electrons are packed as closely as possible, star does not contract further, and temperature and luminosity decrease in time.
What is the Chandra limit?
The maximum possible mass for a white dwarf = 1.4 M@
Which kind of star must collapse to a smaller size and higher density?
A more massive burned-out star
Which objects are highly collapsed?
Neutron stars and black holes
What is an open star cluster?
a cluster of young, recently formed stars found in the disk of a Galaxy where most stars are near the main sequence.
What is the age of an open star cluster?
2 x 10^7 years
What is a globular star cluster?
a cluster of only old stars (greater than 10^10 yr) found in the bulge/halo of our Galaxy, with almost no stars present on the upper main sequence (M>M@).
What is a main sequence turnoff?
the location in the H-R diagram of stars just becoming red giants. The younger the cluster, the higher the mass of the turnoff stars.
What is an equinox?
An event that occurs twice a year where the day and night have equal length. The sun is overhead at equator on equinoxes.
When do equinoxes occur?
The autumnal (fall) equinox occurs around Sept. 22nd and the vernal (spring) equinox occurs around March 21st.
When is the day longer and when is the day shorter during the year?
The day is longer in the summer and shorter in the winter.
In which direction does the sun rise and set?
The sun rises due East and sets due West.
What is a solstice?
An event that occurs twice a year when the Sun reaches its highest or lowest excursion relative to the equator.
When do solstices occur?
Summer solstice (LONGEST DAY OF THE YEAR) occurs around June 21st and winter solstice (SHORTEST DAY OF THE YEAR) occurs around Dec. 21st.
Where does the Earth’s axis point during summer solstice?
It points most TOWARD the Sun.
Where does the Earth’s axis point during winter solstice?
It points most AWAY from the Sun.
During the evolution of a high mass star (M>10M@), nuclear fusion produces elements up to…
Iron (but the core does not undergo fusion, it occurs in the shells surrounding the core)
In a high mass star (M>10M@), the core is the size of…
the Earth
When the core mass of a high mass star exceeds 1.4M@ (Chandra limit), it becomes…
Unstable
What is a supernova and how does it occur?
The death of a massive star. It occurs when gravity overcomes outward electron pressure in iron core, causing rapid collapse.
Electrons and protons combine in a supernova and form…
Neutrons (in nucleus of an atom), and then a pulse of neutrinos (very low mass particles) are produced.
What stops the core’s collapse in a supernova and when?
Neutron pressure, when the core is 20 km (12 mi) across. The pressure balances gravity again in the core.
Describe the supernova’s blast.
The core bounces outward and energy is released by its collapse, which causes a shockwave and the rest of the exploded star to travel out.
What kind of elements are produced in a supernova blast?
Elements heavier than iron are produced in a supernova blast.
What kind of elements are produced in massive stars?
Elements heavier than Hydrogen and Helium.
What do supernova blasts return Hydrogen and Helium to?
Interstellar gas
What was Supernova 1987A?
The closest supernova observed in the past 400 years.
Where was Supernova 1987A observed, when, and how far away was it?
It was observed in Large Magellanic Cloud (LMC) in Feb. 1987 and it was 160,000 ly away.
Describe the Supernova 1987A.
It was a blue supergiant that exploded. Its luminosity increased by a few 1,000x and had three bright rings which provided evidence for the hourglass wind flow of a doomed star.
What did the Chandra X-ray observatory find out about Cas A supernova?
That its remnant was an expanding cloud of hot gas, seen about 300 years after the supernova, about 10 ly in radius.
What is a type of stellar remnant from a supernova blast and describe its structure?
Neutron star, ultra-collapsed core, Mass is equal or greater than 1.4M@, Radius is 10 km or 6 mi, It is made of tightly packed neutrons.
True of false: Neutron stars can emit all forms of electromagnetic radiation: from radio to gamma-rays.
True
Describe the pulsar quality of neutron stars produced by supernova blasts.
The neutron star has a pulsing source of radio, visible, or x-ray. It was discovered by Bell in 1967. The pulse periods were 0.0014 sec to 10’s of sec., up to 700 flashes per sec. The first explanation was alien signals.
What is the lighthouse effect/model for pulsars?
When radiation is beamed along the neutron star’s magnetic axis and the rotational axis and the magnetic axis of the neutron star are not aligned.
Why is rotation of pulsar star after a supernova blast so fast?
Because of the “lighthouse effect”. As its core collapses, the spin rate greatly increases.
What is a pulsar?
A rapidly rotating neutron star (post-supernova).
What is the fastest pulsar spin rate?
700 per sec = 42,000 rpm
How does gas escape from normal stars and form an accretion disk (matter)?
It is attracted by the gravity (spiraling inward in disk) of the collapsed star.
The gas escaping from normal stars is heated to a high temperature by friction and radiates…
Ultraviolet rays and X-ray
What is the x-ray luminosity of neutron star binaries?
10^5 L@
What is the maximum mass for a neutron star?
3 M@
Some x-ray binaries have collapsed stars with…
M > 3M@
If Cygnus X-1 has a visible star that is a B supergiant with strong winds and the collapsed star has a mass of at least 11 M@…
It must be a black hole.
What did Einstein’s Theory of General Relativity (1916) say?
That nothing can travel faster than light and that gravity causes space to curve.
What is a black hole?
An object with such strong gravity that nothing can escape from its vicinity - not even light.
When a star becomes a black hole it collapses to a point of ______ _______ - a _______
Infinite density, singularity
What causes space to curve?
Mass
High mass concentration causes surrounding space to…
Pinch off from rest of universe.
What is an event horizon?
The spherical boundary surrounding the location of a black hole.
Inside the event horizon, the escape speed exceeds…
the speed of light.
The black hole’s mass and its event horizon’s size ________ as material falls in.
Increase
What kind of path do photons follow in curved space?
A curved path (even the Sun bends starlight by a small amount)
The more _____ a star is the more it ______ the surrounding space.
Collapsed, curves
Where are black holes?
In binary star systems (our galaxy contains AT LEAST 10 binaries with a BH) and at centers of galaxies.
What is the average mass of supermassive black holes?
Between 10^6 to 10^10 M@
How do black holes power active galaxies and quasars (an extremely massive and remote celestial object)?
Through gravitational energy release
Stars with M < 10 M@:
- white dwarf
- M (of a white dwarf) < 1.4M@
- R (of a white dwarf) = R (of Earth)
Stars with M > 10 M@:
- neutron star
- M (of NS) < 3M@
- R (of NS) = 10km
Stars with M > 25M@:
- black hole
- M (of BH) > 3M@
- R (of BH) = 3km x M (of BH)
What is the Milky Way?
Our galaxy, a large spiral galaxy (with about 10^11) stars, the Sun is in our disk.
Can you see more stars looking through the disk of our galaxy or looking out of the disk? Why?
You can see more stars looking THROUGH disk than out of disk because the path through the disk is longer.
Why is the Milky Way best viewed in infrared light?
Because dust blocks the view in visible light. Dust emits longer wavelength in infrared and lower temp. (cooler). Stars emit shorter wavelength in infrared and higher temp. (hotter).
How is the disk of the Milky Way galaxy shaped and what is its structure?
Like a frisbee. It contains stars (young and old), gas, and dust in a spiral pattern and is 2,000 ly thick.
Since the Milky Way galaxy orbits circularly, what is the orbital period of the sun and how many orbits has it completed?
The orbital period of the sun is 2 x 10^8 yr. The sun has completed 25 orbits.
Describe the evolution that occurs in the disk of the Milky Way.
Star formation is ongoing. Each stellar generation adds “metals” (elements heavier than Hydrogen and Helium) to interstellar gas, from supernova explosions.
Why do young stars have higher metal content in their atmospheres than old stars?
Because young stars form from enriched gas due to supernova explosion.
What are the spheroidal components of the Milky Way?
The nucleus, bulge, and halo (contains no gas and dust, so star formation isn’t occurring - old stars only).
True or false: It was once thought that the sun was at the center of the galaxy.
True. Shapley discovered this.
What is a standard candle?
An object of known luminosity.
A standard candle measures _____ _______.
Apparent brightness
Apply the inverse square law to get distance:
B = L/d^2 (discovered by Shapley)
Where is our galactic center located and how far is the sun from it?
In Sagittarius. About 25,000 ly from the galactic center.
How is the galactic nucleus best observed and describe the region.
It’s best observed using infrared and radio telescopes. It is a crowded, active and dense region where nonstellar and gamma-ray emission can be observed.
What does the galactic nucleus contain?
A supermassive black hole
High speed stellar orbits (faster as it gets closer to black hole) indicate the mass of the black hole is…
4 x 10^6 M@
What produces gamma-rays near a black hole?
electron-positron annihilation
There is a large amount of dark mass in the halo due to…
the stars in the halo having high speeds.
How far does the halo extend in relation to the size of the disk?
It extends at least 10x the size of the disk.
What kind of dark matter could the halo possibly contain?
- “Jupiters” (M = 0.001 M@)
- brown dwarfs (M = massive compact halo objects (very low mass stars or collapsed stellar remnants)
What are some examples of MACHOs?
white dwarfs, neutron star, or black hole
How can MACHOs be detected?
Through gravitational lensing (when an object passing between a light source and the observer bends the light and it temporarily appears brighter)
As much as __% of dark matter is still not known. Most of it is supposed to be in the form of ______.
90, WIMPs
What causes each star to orbit the globular cluster in a rosette pattern?
Gravity
Describe cluster evolution.
Close encounters occur between pairs of stars, some stars are kicked into core or halo, others collapse and shrink to become denser.
How are close binary stars formed?
By gravitational interactions in the core.
What makes and x-ray binary?
If one of the two stars is a white dwarf, neutron star, or black hole.
What can eject binaries and singles from a cluster and what happens to them after they are ejected?
Gravitational interactions between them. They join the galactic halo.
What kind of stars are most cluster stars?
Low-mass, red, old stars
What are blue stragglers?
Small numbers of higher-mass, blue, apparently young stars.
What does the stellar evolution theory say about blue stragglers?
That they should not exist now and that they should have evolved to red giants then white dwarfs.
What is the theory for blue stragglers and where did the Hubble space telescope detect them?
The theory is that they were formed by stellar collisions. The telescope detected them in cluster cores, where collision rate is higher.
Is there a large black hole at the center of globular cluster M15?
Possibly. Black holes would cause a sharp increase in stellar speed toward its center but it can be measured by a central concentration of white dwarfs and neutron stars, so it’s not positively a black hole.
What are nebulae?
faint, fuzzy, cloud-like objects in the sky that don’t move relative to fixed stars, thus are not comets.
What is Meissier’s List (1781)?
A list of the 103 brightest nebulae. Ex.) M1 = Crab Nebula, M31 = Andromeda Nebula, M51 = Whirlpool Nebula.
What is an island universe?
A self-contained stellar system outside the Milky Way - another name for a galaxy.
What is a hypothesis about nebulae in the island universe model?
That most nebulae are galaxies.
When, by whom, and how was the island universe model confirmed?
In 1924, by Hubble, when he found Cepheid variable stars in M31 (Andromeda nebulae).
What is a cepheid variable?
a star that varies between a larger, brighter state and a smaller, denser one
What is a cepheid variable’s period?
The time between its luminosity peaks
How do you find a cepheid variable’s distance?
Use the inverse-square law (B=L/d^2) to find distance from its apparent brightness and luminosity.
What kind of luminosity do the brightest cepheid variables have?
L > 10^4 L@
Where is M31 (the Andromeda Galaxy)?
It lies well outside the Milky Way at the distance 2 x 10^6 ly.
What is a spiral galaxy and what are two examples of one?
It is a galaxy made up of a disk w/ spiral pattern and spheroid (and bulge and halo), with young and old stars. Two examples are the Milky Way and the Andromeda Galaxy (M31).
What is an elliptical galaxy?
It is a galaxy with a smooth, symmetric structure; spheroidical only (like football); old stars only.
What is an S0 (S-zero) Galaxy?
It is a galaxy with a disk but NO gas, dust, or spiral structure (unlike all others). Old stars only.
What is an irregular galaxy?
It is a galaxy with asymmetric structure, high gas/dust, vigorous star formation. (Ex.) Magellanic clouds)
Most galaxies are found in ______ or ______.
Groups, clusters
What is the range of size for galaxy clusters?
10^6 to 10^7 ly across
What is the local group?
The nearest 30 galaxies to us.
What is the structure of the local group?
Mostly irregular and elliptical galaxies. MW and Andromeda (M31) are on opposite sides falling towards each other, may merge into an elliptical in 5 x 10^9 yr.
True or false: Stars collide in galaxy collisions.
False. They are too far apart.
True or false: Gas clouds collide in galaxy collisions.
True. Due to their large sizes. Gas can be heated and may excape from galaxies. THIS CAN TRANSFORM SPIRALS INTO S0s!
Low-speed galaxy collisions can cause galaxy mergers in _____ ______.
Small groups
High-speed galaxy collisions in large clusters do not tend to result in ______.
Mergers
Collisions can compress gas clouds in galaxies, causing _____ _______.
Star formation
In the galaxy NGC 6240 (which was formed by a merger), there are 2 supermassive black holes that are about 3000 ly apart. In about _____ million yr they will _____, producing strong _____ waves.
100, merge, gravity
What accelerates the speed of galaxy clusters?
Gravitational pull
The higher the speed of a galaxy cluster, the ______ the mass of the cluster.
Greater
Less than ___% of the mass of a cluster is in visible parts of galaxies and hot gas.
20
The other 80% of the mass of a cluster is ____ ____, thought to be WIMPs.
Dark matter
What is a supercluster and how is it produced?
A group of galaxy clusters, produced by gravity.
What are three features of a large-scale distribution of galaxies?
Filaments, voids, and walls
What are filaments?
Apparent strings of galaxies in space
What are voids?
Empty regions in space
What are walls?
Sheets of galaxies in space at the edges of voids.
How do you find the distance of galaxies?
Use standard candles and apply the inverse square law (B=L/d^2)
What are 5 examples of standard candles?
Cepheid variable, supergiant stars, globular star clusters, galaxies, and supernovae.
More _____ galaxies require more luminous candles.
Distant
What is the Hubble Law (1929)?
When Hubble measured Doppler shifts of absorption lines in galaxy spectra and found that all but nearest galaxies RECEDE from Milky Way and have RED SHIFTS.
The more distant the galaxy, the ______ it recedes.
Faster
A galaxy is at a distance of 100 MPC, what is its speed?
v=H0(hubble constant=71)d
v=71 x 100MPC
v=7100 km/s
How do you measure v when determining the Hubble constant?
You measure v from doppler shift (see formulas), then calculate d=v/H0.
What is the interpretation of the Hubble law? What does it mean?
That the universe is expanding. The distances BETWEEN galaxies all increase in time. It started with the Big Bang about 14 x 10^9 yr ago.
Other than the Sun, how many planets have been confirmed around stars and how close are they located to their parent star?
1000, and very close (<2 AU)
The gravity of planets causes stars to _____, which is measured by the ______ ______.
Wobble, doppler shift
The more _____ the planet and the ______ it is to the parent star, the larger the wobble of the star.
massive, closer
What makes a planet habitable?
If it is the right mass (too high and star’s lifetime is too short, too low and too cold), solid surface, atmosphere not too thick or thin (too thin - cannot maintain liquid water - Mars; too thick - runaway greenhouse effect - Venus)
Each star has a _______ zone.
Habitable
Venus and Mars are not in habitable zone. Venus is too ____ and Mars is too _____.
Hot, Cold
What is the Kepler Mission (2009-)?
A spacecraft based telescope helped to discover planets that orbit other stars due to sensing the dip in brightness that the causes by being in front of its parent star.
It is estimated that ___ billion Earth-sized planets may be within the habitable zones of the Sun.
40
Life on Earth is based on ______ ______.
Organic molecules
What are two examples of complex organic molecules?
Formaldehyde and Ethyl Alcohol
_____ may have seeded young Earth with organic materials.
Comets
What is the Miller-Urey Experiment?
An experiment that showed that complex organic molecules form in simulated primordial Earth conditions. (Did NOT create life or DNA)
What does the Drake Equation do?
It considers factors that determine the likelihood of other intelligent life in our Galaxy. (Number of intelligent civilizations in our Galaxy = R x I x U x…etc. = 1 to 1000)
If the human race intends to find out if alien life forms exist, what are two conclusions we can come to at the present time?
That radio is the best means of contact (possible planets would be too far away and take 200 ly < to get there) and it is highly unlikely that aliens have visited Earth.
What is cosmology?
The study of the structure and evolution of the universe. (All galaxies moving away from each other)
What are properties of the universe’s expansion?
There is NO center or edge, it obeys Hubble Law, and more distant objects recede FASTER.
As the universe expands, photons are _____. Wavelengths get _____ and photons are ___shifted.
Stretched, longer, red. The more distant the object is the redder it is and the higher recession velocity it has.
The universe can be traced to a state of infinite density about ___ Gyr (10^9 yr each).
13.7 (extremely accurate)
Before galaxies and stars formed, the universe was filled with ___ ____ and ______.
Hot gas, radiation
What is the Cosmic Microwave Background?
An experiment that detected microwaves from all directions in the sky and won the 1978 Nobel Prize. It proved the Big Bang happened.
What is the geometry of the universe and its content?
Flat and infinite, made up of normal and dark matter (WIMPs), and dark energy.
The universe is expanding ever more rapidly, so will it reach a maximum and then start shrinking back?
Probably not. It will probably just keep expanding.
