AST 180 Test 2 Reading & HW Qs
Replace the light bulb and gas cloud in the figure below with an O star and a cloud of hydrogen atoms. If you observe an emission nebula from the combination, where are you located in the figure?
2
How far is the Sun from the Center of the Milky Way?
26,000 ly
What is the minimum safe distance for life on Earth to be from a supernova explosion?
260 ly
what is the distance to the star Spica?
260 ly
If we measure an apparent magnitude, m, of +10 for a star, and a distance of 200 parsecs for the star, what is the absolute magnitude, M, of the star?
3.5
We measure the parallax of a star to be 0.2 arc sec. What is the distance to the star in parsecs?
5.0
The center of our Galaxy has a black hole with enough mass inside to make almost 4 million Suns! How do astronomers think a black hole could have formed and acquired so much mass?
A large cloud of gas near the center of the Milky Way could have collapsed directly to form a black hole when the Milky Way was just beginning to take shape A massive star at the center of the Milky Way might have exploded to leave behind a seed black hole, and then a dense cluster of stars might have collapsed into the seed black hole. Once a seed black hole exists at the center the Milky Way, it grew over the next several billion years by devouring nearby stars and gas clouds in the crowded central regions. It could grow by merging with other black holes
H-R diagrams for four clusters are shown in the figure below. Which cluster is oldest?
C
In the Figure below, which letter would represent our Sun?
C
In the Figure, which letter represents the star with the coolest surface temperature?
D
Which of the following is NOT true about Population I stars?
Globular clusters are Population I stars
Where do you find the youngest stars in the Milky Way?
In the disk
Stars in the neighborhood of the Sun are dominated by
M V stars
Which star spends the most time on the Main Sequence?
M star
One of the most important observations in the history of astronomy was the one by Edwin Hubble that established that there are other galaxies, quite removed from the Milky Way. How did Hubble show this?
Observing a Cepheid variable in a nearby galaxy and using it to get the distance
Which star below will eventually go supernova and is closest to the Earth?
Spica
William Herschel thought that the Sun and Earth were roughly at the center of the great grouping of stars we call the Milky Way. Today we know this is not the case. What was a key reason that Herschel did not realize our true position in the Milky Way?
The dust that extends throughout the disk of the Galaxy only allowed Herschel to see the small part of the Milky Way that surrounds us
What leads astronomers to conclude that the proto-galactic cloud (the cloud from which our Galaxy formed) was roughly spherical?
The oldest stars in the Galaxy (Population II stars, globular clusters) form a spherical halo around the Galaxy; they outline the original shape of the cloud that gave the Galaxy birth
Recently, astronomers have observed stars and other objects that orbit the center of the Milky Way Galaxy farther out than our Sun. These objects move around the center of the Milky Way faster than we do. How do astronomers think such an observation can be explained?
There must be a great deal of invisible dark matter outside the orbit of the Sun whose gravitational pull explains the faster motions we see out there
Which of the following is NOT true of a neutron stars?
They are about the size of the Earth
The first astronomer to show that spiral nebulae (today called spiral galaxies) have large Doppler shifts was
Vesto Slipher
What have we learned from the work of Harlow Shapley and others about the location of the Sun in the Milky Way Galaxy?
We are in the disk of the Galaxy, about 3/5 of the way from the center
Deep inside a black hole (and hidden from our view) is the compressed center, where all the "stuff" of the star goes. Astronomer call this central point
a singularity
The Sun's radius is relatively constant because
at each layer in the Sun, the outward gas pressure is balanced by the inward weight of the layers above
Which of the following statements about the different types (shapes) of galaxies is correct?
collisions and mergers between galaxies can sometimes change a galaxy's type (shape)
The brightest stars in the night sky are bright because
despite their large distances, they are huge stars and are therefore intrinsically very bright.
Which of the following was NOT done by Edwin Hubble?
discovered the relationship between period and luminosity of a cepheid variable
A Main Sequence star
is an ordinary star converts hydrogen to helium in its core
As a part of a dark cloud evolves from a protostar to a Main Sequence star
it gets smaller in volume it gets hotter
Which of the following is NOT a characteristic of a spiral galaxy?
it has little or no Population I stars
Today, astronomers find compelling evidence that the energy source of the quasars and active galaxies is
matter falling toward a supermassive black hole at the center of a galaxy
Wearing a watch, you volunteer to go on a mission to a black hole in a spaceship that has powerful rockets. You are able to orbit the black hole and stay a little distance outside of the event horizon for two weeks. Compared to watches on Earth, your watch near the black hole will run:
more slowly
Upon returning from the black hole to the Earth in the above question, you friends and family will be
much older than you
Why do astronomers think that there are fewer quasars today than there were billions of years ago?
quasars are seen when the supermassive black hole at the center of a galaxy has a lot of "food to eat" (stars and gas) and the amount of available food tends to decrease with time
What makes astronomers believe that the energy source in quasars is only a few light months across (the distance light travels in a few months)?
quasars show variations in their energy output that have a period of a few months
The new instrument that made it possible for Edwin Hubble to demonstrate the existence of other galaxies in the early 1920's was:
the 100-inch (2.5-meter) telescope on Mount Wilson
Absolute Visual Magnitude is
the apparent magnitude of a star observed from a distance of 10 pc
After a 0.8 to 2.0 solar mass star exhausts the helium fuel in its core which of the following happen
the carbon and oxygen core begins to collpase the core grows hotter and ignites a helium burning shell around the core the star once again swells into a giant star the star blows off a significant part of its outer material to form a planetary nebula
The Sun's tremendous energy comes from
the conversion of hydrogen to helium at the center of the Sun
The source of the Sun's tremendous energy is
the conversion of hydrogen to helium in its core
A friend of yours who is a science fiction fan hears you talk about the fact that astronomers now believe that the mechanism for the large energy output of quasars involves a supermassive black hole. He challenges you, saying something like "Oh come on, every science fiction fan knows that nothing, not even light, can escape from a black hole! How can a black hole be an energy source?" How would you respond to his objection?
the energy we see from quasars comes from regions where matter is falling in; these regions are still outside the event horizon
The boundary around a black hole where everything inside the boundary is trapped, and nothing can get out to interact with the rest of the universe, is called
the event horizon
Edwin Hubble was able to show that (with the exception of our nearest neighbors) the farther a galaxy is from us, the
the faster it is moving away from us
When astronomers make counts of how many quasars there are at different distances from us, what do they find?
the largest number of quasars can be seen at about the distance corresponding to a time when the universe was only 20% its current age (a time when the universe was still young)
What causes a star to swell into a red giant?
the onset of the conversion of hydrogen to helium in a shell around the helium core
If quasars often resemble little blue stars, what was it about them that so surprised astronomers when they were discovered?
their spectral lines were at first hard to recognize and then turned out to have large redshifts
After several decades of observation, astronomers have concluded that quasars are
very powerful and compact sources of energy at the centers of distant galaxies
What is a neutron star? What are the properties of neutron stars?
•A supernova explosion of high mass star blows away its outer layers. •The central core of star collapses into a compact object of a couple of solar masses •Mass of 1.4X to 3X the mass of the Sun in a sphere about 20 km in diameter •Nuclei ripped apart into protons and neutrons •Protons and electrons combine to form neutrons •Core becomes a gigantic ball of neutrons •Neutron gas keeps gravity from crushing the neutron star •A teaspoon of a neutron star material on Earth would weight about the same as Mt Everest •Star rotates on its axes •Star cores rotate on its axes •In collapsing from supergiant core to a neutron star, the core shrinks tremendously in size as it keeps rotating (1967)
White Dwarf Properties
•Bare carbon, oxygen core of medium mass star at the center of expanding planetary nebula •Surface temperature about 25,000 K (hot) •Luminosity about 1/100 luminosity of sun (faint) •Mass of a main sequence star like the Sun in the same volume as the Earth •Extremely dense material •Chunk of white dwarf the size of a beach ball would outweigh an ocean liner •Made of carbon nuclei, oxygen nuclei, and electrons •Pressure of exotic electron gas keeps gravity from crushing white dwarf
How will the Sun evolve after it leaves the main sequence?
•Medium mass stars (like the Sun) end up as white dwarfs •High mass stars end up as neutron stars or black holes
What is a blackhole? What are the properties of blackholes?
•Neutron stars cannot exist with masses > 3X the mass of the Sun •No mechanism can stop the collapse of a compact object with a mass > 3X the mass of the Sun •The compact object will collapse into a single point - a singularity, an "object" with ZERO volume and infinite density A black hole •The material collapses under its own weight forming a place with zero volume and infinite density, a singularity •Space-time ceases to exist •The laws of physics break down •We don't have the physical understanding or mathematical tools to describe the singularity•Close to the event horizon, the force of gravity on astronauts feet becomes much larger than force of gravity on the astronauts head •The astronaut is stretched out and ripped apart even before crossing the event horizon •Spaghettification•No light can escape a black hole •Impossible to observe a single black hole •Look for unseen compact objects as part of a binary system •We can estimate the mass of compact object from orbital period and radial velocity (Doppler effect) of visible star •Compact Object Mass > 3X Mass of Sun
