astronomy1020
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- If nuclear reactions ceased to occur in the Sun's interior, how long would it take for the Sun to shrink?
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20 million years
Estimated by Kelvin and Helmholtz. Sun must have another source of energy besides gravity. - T/F? Results of the first experiemtns to measure neutrinos from the Sun agreed with predictions from theory of the Sun's interior
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False.
Experiments observed flux of neutrinos was 1/3 of prediction from theory. Neutrinos can switch among 3 types according to orig. experiment - T/F? Mos tof the 25 nearest stars are invisible to the naked eye
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False.
Stars have very low luminosity compared to Sun. Low mass, main sequence stars called "red dwarfs" - Which star on H-R diagram has the smallest mass?
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lower right corner.
Low mass, main sequence called red dwarf. Same as the Sun. - Pick the star in the H-R diagram that might have this (photo of layers) central structure.
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Upper right.
AGB star that is about to become a supernova. - Which location on an H-R diagram indicates a star that was originally most massive?
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Lower left.
Because it's a white dwarf and most massive stars always evolve first. White dwarf is the most evolved stage - Is a classical nova explosion more more likely to occur in a binary system with white drawfs than a younger system?
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Yes.
Classical nove come from white dwarfs in binary systems - What factor determines whether a system will become a thermonuclear supernova?
- The composition of the donor star: if it is made of helium, it will deposit helium onto the white dwarf surface causing an explosion that would cause the entire dwarf to explode.
- T/F? Pulsars were predicted before they were found.
- False.
- What did Baade and Zwicky predict in regards to neutron stars?
- They predicted that supernovae would produce neutron stars.
- What did Pacini predict in regards to neutron stars?
- He suggested the source of energy in the Crab Nebula was a rotating magnetic neutron star.
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The energy release in an X-ray burst is:
a. less than
b. about equal to
c. greater than
the energy released in a classical nova explosion. -
a. less than
surface area of neutron star is much less than white dwarf, gravity on surface of neutron star is greater than white dwarf. -
A black hole with mass equal to that of Earth will have a radius roughly equal to that of:
a. ball on a ball-point pen
b. a marble
c. a basketball
d. city of Boulder - marble
- T/F? Black holes were predicted long before Einstein's theory existed.
-
True.
John Michell, 1784 - T/F? It is probably impossible for a star to collapse into a black hole having less than 2 solar masses.
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False.
If core has mass less than 1.4 Suns, it will stop collapsing and end up as white dwarf. If it has mass less than 2-3 solar masses, neutron star. - What is the limiting factor for the angular resolution of ground-based telescope?
- Defocusing due to turbulence of earth's atmosphere.
- What is the limiting factor of Chandra X-ray telescope?
- The smoothness of the mirror
- What is the limiting factor for the Spitzer infrared space telescope?
- Diameter of the mirror (diffraction limit)
- Main sequence "red dwarf" star with mass 1/3 of Sun and radius 1/3 of sun has surface temp of 3000K (1/2 of Sun). Estimate or calculate ratio of luminosity of this star compared to Sun
- L=4piR^2T^4
- List 2 observational clues (before pulsars) that Crab Nebula contained neutron star.
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1. moving "wisps" around central star.
2. fraction of radio emission was randomly fluctuating. Indicated that Crab contained radio source as well as extended one. - What theoretical prediction provided the key test showing that pulsasrs were actually rotating magnetic neutron stars?
- Thomas Gold predicted that the pulsation rate would slow down as rotating neutron star lost energy.
- State two main observational facts leading to conclusion that Cygnus X-1 must contain a black hole.
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1. Doppler shifts of companion star indicated that it was orbiting an onject having mass greater than 7 suns.
2. xrays coming from cygnus varied on timescales less than one millisecond, indicating size of xray source was smaller than earth - There is no known stable object that is more massive than 3 suns and smaller than the earth except for ________.
- a black hole
- In denver, NPR FM station has frequency of 90.1 MHz (9.01 x 10^7 Hz) What is the wavelength of these radio waves? How long does it take for the radio waves to arrive at your current location?
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Wavelength is 3.33 meters. (wavelength = c/frequency) c=3.0 x 10^8 and f=(9.01 x 10^8)/(9.01x10^7)-3.33 m
It takes .00016 seconds for these to reach Boulder. (Denver is approx 30 miles from Boulder. 30 miles=48km =4800m. distance/speed=time. - What is the most challenging technical problem in building an infrared telescope in space?
- The biggest problem is super-cooling the telescope with liquid helium so that ther is no interference from camera radiation.
- Star A is redder than star B. An astronomer says, "I think that star A and star B actually have the same temperature, but star A only appears redder because it lies behind an interstellar dust cloud." How can she know that?
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Color of star comes from temperature, which can be determined by the star's spectrum. If the two stars exhibit the same qualities, they have the same temperature.
However, star A looks more red because the dust cloud is absorbing the blue light that the star is emitting. Therefore, both stars can have the same temp, while having slightly different color. - What is plotted in a H-R diagram?
- stars are plotted, x-coordinate is temperature, y-coordinate is visual luminosity
- What properties of a star must astronomers observe to generate an H-R diagram?
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Measure paralax
Brightness
(plug into inverse square law)=Luminosity
Temperature - How is the Hipparcos satellite useful for plotting H-R diagrams?
- Satellite enabled scientists to measure paralaxof star without blurring cause by Earth's atmosphere. Accurate.
- Suppose that you observed the color of a star and its magnitude, what assumption would you have to make about the star to estimate its distance? How?
- By observing the color you would estimate the temperature, but would have to assume that there is no interstellar dust prohibiting the star's light from passing through.
- Of the 25 nearest stars in the sky except the Sun, what is the average distance in light years?
- 9.396 light years
- Of the 25 nearest stars in the sky except the Sun, which ones are visible to the naked eye?
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For a star to be visible, it must have a magnitude of 1.5 or less.
Cirius
Alpha Cen A
Alpha Cen B
Procyon A are visible. - Of the 25 nearest stars, how many are hotter than the sun?
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4 stars:
Sirius A
Sirius B
Procyon A
Procyon B - Which spectral type abbreviation is the hottest? The coolest:?
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O - hottest
M - coolest - What is the average distance of the 25 brightest stars in the sky?
- 314.932 light years
- Of the 25 brightest stars in the sky, how many are hotter than the sun?
- 18
- What are the orbits of stars like when the masses are equal?
- The orbits parallel each other
- How do the orbits of two stars change as you change the mass ratio?
- As the mass of a star increases, its orbit becomes smaller, vice versa. The larger its mass, the slower it moves by a factor of 10.
- How long will a main sequence star 1/10 as massive as the Sun continue to burn?
- Because of E=MC^2 and energy released by the fusion of hydrogen into helium, the Sun will burn for 10^10 years.
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We know that Kelvin and Helmholtz were wrong in their thoery about the Sun's luminosity and lifetime.
Is their theory completely useless? Why or why not? -
It is not useless because it predicts what happens when a star begins to change and burns out faster as it runs out of hydrogen in the core.
Because of this, core shrinks because it loses energy in form of heat-- 20 million years. - What happens to a star when it consumed all the hydrogen in its core?
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It begins to shrink. Gravitational contraction causes star to become hotter, which causes hydrogen fusion reactions in shell around core.
Becomes more luminous, radiation cannot escape as fast as produced.
Envelope expands and star becomes red giant. - The most luminous star known was discovered with an infrared camera on the Hubble. Why wasn't it discoverred at optical wavelengths?
- because the star was blocked by a dust cloud which makes it hard to see with optical lense.
- Describe interior stucture of a red giant.
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High density in core, lower in outer
Hot heluim core, surrounded by shell of burning hydrogen, covered by envelope of expanding gas due to shell heat.
100+ times bigger than it was as an MS star.
Between 10^7 and 10^8K, any hotter and it would become a horizontal branch star. -
How many millions of years does it take for a star of mass 2x sun to become a red giant?
7x ?
25x ? -
2x: 2032.05 million years
7x: 57.09 million years
25x: 8.27 million years
Stars with greater masses burn faster because they are like SUVs --consume fuel quickly and inefficiently -
Triple-alpha reaction:
Describe exterior appearance and
interior structure. -
Helium flash-- star becomes horizontal branch star.
Core fuses helium into carbon
Hydrogen burning shell / outer layers
Core expands, pushing shell out
Lowers temp.-- luminosity
Layers shrink-- becomes smaller
Surface temp shrinks - What does a star become after a triple-alpha reaction has occurred?
- an AGB star
- What is the difference between a red giant star and an AGB star?
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Red giant has intert helium core with burning hydrogen shell.
ABG has inert, very dense carbon/oxygen core, burning shell of hydrogen, and hydrogen envelope.
Appearance is similar, yet ABG slightly more luminous. - Suppose that you found a star cluster in which there were plenty of main sequence stars having photospheric temperature less than 10,000K but none with greater temperature. How old is this star cluster?
- If a star cluster with many main sequence stars doesn't have any stars with temp. greater than 10,000K, relative age would be around 143 million years old.
- H-R diagram of the globular cluster M3 contains red giants (both RGB and AGB) and horizontal branch (HB) stars, Rank these in order of mass.
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Since it is a globular cluster, we can assume stars are all similar age. AGB have already evolved through RGB and HB phases, so they have shortest main sequence lifetimes, followed by stars currently in HB and RGB.
Current mass is reverse order of original masses:
RGB
HB
AGB - What kind of star is Sirius B? How do we know its mass and radius?
- Sirius B is a part of a binary system of three stars. Binary systems are important b/c we can measure masses in relationship to centrifical force and gravity.
- What is the attractive gravitational force?
- Proportional to the product of two masses divided by the separation distance.
- Who was Chandrasekhar?
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Chandrasekhar was a grad student at Cambridge in the 30s. Invented theory in which white dwarf star with same mass as Sun would have a radius less than that of Earth.
The greater the mass of this star, the smaller its radius-- won Nobel Prize - Why did Chandrasekhar move from England to the United States?
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Sir Arthur Eddington, also at Cambridge, publicly ridiculed his results.
If a white dwarf weighed more than 1.4x the sun it would collapse under own weight - How is a thermonuclear supernovae similar to a classical novae?
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-burning, fusing surface layer
-explosive ending
-begin as white dwarfs - In what ways are thermonuclear supernovae and classical novae different?
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-donor star transfers pure helium in thermonuclear, hotter.
-higher temp and pressure required to ignite helium, so thicker layer accumulated on surface
-thermo: much more violent explosion - Is it possible for a core collapse to occur in star cluster like Pleiades?
- Yes, because it is an open cluster and there are few white dwarf stars.
- Is it possible for a core collapse to occur in a globular cluster like M3?
- No, there are many white dwarfs, which means many stars ae less than 8 solar masses. Will remain white dwarfs.
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What led to the logic that the heavy elements on Earth come from supernova explosions?
What observations of SN1987A supported this? -
Curve of binding energy state that light elements (carbon and oxygen) can fuse into heavier elements. Stars become white dwarfs before supernova explosion occurs (core is carbon and oxygen) these elements fused in high temps.
SN1987A: light from supernova faded at same rate that radioactive Cobalt-56 decays into stable Iron-56. When gamma ray photons from SN debris were measure, they had exact same energy as Cobalt-56 in experiment. - What was the proof that pulsars are neutron stars?
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Pulsar is a remnant star of a core collapse supernova that exploded many years ago.
Thomas Gold predicted that pulsars emit radio pulses b/c they are rotating neutron stars w. strong magnetic fields. Observation of Crab Nebula proved that we can see the pulsation rate of a star slow down as magnetic fields convert energy into particles. - Why do all known neitron stars have masses equal to approx. 1.4 solar masses?
- 1.4 solar masses is the Chandrsekhar limit: the point at which white dwarfs collapse and become a neutron star.
- How are x-ray bursters similar to classical novae?
- Both are the result of a build-up of hydrogen accreted on the star, however x-ray bursters occur on neutron stars and classical novae occur on white dwarfs.