Astro exam 3

When does a star leave the Main Sequence?

"When stars run out of hydrogen, they begin to fuse helium in their cores. This is when they leave the main sequence. High-mass stars become red supergiants, and then evolve to become blue supergiants. It's fusing helium into carbon and oxygen"

What are the possible ultimate fates of low and high mass stars?

- "A massive star ends with a violent explosion called a supernova." - "These very low mass stars will eventually end as white dwarfs made of helium."

What creates a supernova? What is meant by the "iron catastrophe"?

- "A supernova happens where there is a change in the core, or center, of a star. ... One of the stars, a carbon-oxygen white dwarf, steals matter from its companion star. Eventually, the white dwarf accumulates too much matter. Having too much matter causes the star to explode, resulting in a supernova." - "The Iron Catastrophe is an event that shaped planet Earth as we know it, and it happened close to 50 million years ago. It is theorized how the radioactive elements, who were trapped inside Earth, raised the temperature so much that it caused the melting of iron that eventually formed the Earth's core."

What are the defining features and origins of: Brown Dwarfs, White Dwarfs, Neutron Stars, Black Holes?

- "Brown dwarfs are objects with a mass that range between the heaviest gas planets and the lightest stars, which makes them distinct enough to qualify for their own classification." - "White dwarfs are stars that have burned up all of the hydrogen they once used as nuclear fuel. Fusion in a star's core produces heat and outward pressure, but this pressure is kept in balance by the inward push of gravity generated by a star's mass" - "They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf stardensity to that of atomic nuclei." - "Primordial black holes are thought to have formed in the early universe, soon after the big bang. Stellar black holes form when the center of a very massive star collapses in upon itself. This collapse also causes a supernova, or an exploding star, that blasts part of the star into space."

By what process do stars form? How are temperature and pressure related and important in this process?

- "Stars form from an accumulation of gas and dust, which collapses due to gravity and starts to form stars. The process of star formation takes around a million years from the time the initial gas cloud starts to collapse until the star is created and shines like the Sun" - "When the protostar starts fusing hydrogen, it enters the "main sequence" phase of its life. Stars on the main sequence are those that are fusing hydrogen into helium in their cores. ... Its inner layers start to collapse, which squishes the core, increasing the pressure and temperature in the core of the star."

What is the likely formation history of the Milky Way? How is this formation history supported by our observations of what kinds of objects live in the disk vs. in the halo?

- According to ELS, the Milky Way began as a spherical cloud of gas—a protogalaxy—that was born collapsing toward its center. The original gas was poor in metals, and so stars formed as the cloud was collapsing would also be metal poor. -

What is the sun's location within the Galaxy? How was it proven that we were not at the center of the Galaxy?

- Bottom line: The sun is about 1/3 the distance from the center of the Milky Way galaxyto its outer edges. It's located in a smaller spiral arm, between two large arms, called the Orion Arm.

What are the differences between globular clusters and open clusters of stars? How can we use H-R diagrams of star clusters to measure their ages?

- Globular clusters are old clusters of stars that have remained in a gravitationally bound system. ... Open clusters are much younger and smaller than globular clusters. They are the recent birthplaces of new stars, which form out of clouds of dust and gas, and contain only hundreds or thousands of stars - because we know how a star's mass determines how quickly it burns its fuel and where it falls on the diagram, we can use the main sequence turnoff to figure out the age of the cluster. For example, a bright O star will live about a million years. A G-type star like the Sun will live about 8 billion years.

What was the subject of the "Great Debate"? Who won the debate and what were the main points of contention?

- It concerned the nature of so-called spiral nebulae and the size of the universe; Shapley believed that distant nebulae were relatively small and lay within the outskirts of Earth's home galaxy, while Curtis (island) held that they were in fact independent galaxies, implying that they were exceedingly large and distant. - Shapley "won" the debate but, ultimately, Curtis was right. Van Maanen's measurements were in error. And it turned out there were two types of novae — the other being the much brighter supernovae.

What is "metallicity" and why can we use it to determine a star's age? What kind of observations are used to measure a star's metallicity? What elements are astronomer's talking about when they say "metals"?

- Metallicity refers to the abundance of different chemical elements in the star. This information allows them to refine the age of the star. ... "That would tellus more about Galactic chemical evolution, based on how different elements built up over time as starsformed in our galaxy, the Milky Way." - - Astronomers refer to all the chemical elements heavier than hydrogen and helium as 'metals', even though this includes elements such as carbon and oxygen which are not considered metals in the normal sense.

What are the major members of the Local Group? What kind of galaxies are they? What is their eventual fate?

- One of the most prominent members of the Local groupis M31, the Andromeda Galaxy. It has two small satellite galaxies, M32 and M110. Also prominent in the local group is the Triangulum Galaxy (M33), Leo I, and NGC 6822. - The three largest members of the group are our Milky Way (second-biggest), the Andromeda galaxy(biggest) and the Triangulum Galaxy. The other galaxies in the Local Group are dwarf galaxies, and they're mostly clustered around the three larger galaxies. -

What is the difference between Cepheids and RR Lyrae stars, when using them as standard candles? What role did Cepheids play in settling the "Great Debate"?

- RR Lyrae variables are much more common than Cepheids, but also much less luminous. The average absolute magnitude of an RR Lyrae star is about +0.75, only 40 or 50 times brighter than our Sun. Their period is shorter, typically less than one day, sometimes ranging down to seven hours. -

How do Elliptical, Spiral, Barred Spiral, and Irregular galaxies differ in their basic appearance? How do they differ in terms of their formation history? What major differences in the gas/dust and stars do we see when comparing Elliptical and Spiral galaxies?

- Some spiral galaxies are what we call "barred spirals" because the central bulge lookselongated - like a bar. ... They have a bulge and halo, like spiral galaxies, but don't have the flat disk of stars. The stars in ellipticals tend to be older. Irregular galaxies have no identifiable shape or structure to them. - -

How do a galaxy's spiral arms keep their shape? What do the spiral arms consist of?

- The idea is that the entire disk of a galaxy is filled with material. As this density wave passes through, it's thought to trigger bursts of star formation. The spiral arms of a galaxy mark where in the galaxy the density wave recently passed, causing new stars to form and burn brightly. - The disk of stars orbiting the bulge separates into armsthat circle the galaxy. These spiral arms contain a wealth of gas and dust and younger stars that shine brightly before their quick demise. How the spiral arms form continues to puzzle scientists.

What would it look like, from the outside, to watch a clock fall into a black hole?

- spagetti

What is a "Standard Candle"? Why is it possible to use Standard Candles to measure distance? What are the basic features of RR-Lyrae stars?

-A standard candle is an astronomical object that has a known absolute magnitude. - For distances which are too large to measure using parallax, astronomers use'standard candles'. Light sources which are further away appear fainter because the light is spread out over a greater area. If we know how luminous a source really is, then we can estimate its distance from how bright it appears from Earth. - RR Lyrae are variable, horizontal branch stars with periods ranging from a few hours to 2 days, and optical brightnesses that typically vary between 0.3 and 2 magnitudes. They lie in the instability strip of the Hertzsprung-Russell diagram and suffer instabilities that cause their size to periodically change.

What are 30 Doradus and NGC 604?

30 Doradus and R136 lie in the LMC - a satellite galaxy to our own Milky Way Galaxy. ... But NGC 604 was so large, it could form enough stars to make a globular cluster. Many young stars from this cloud are visible above, along with what is left of the initial gas cloud.

How are black holes created?

A black hole can be formed by the death of a massive star. When such a star has exhausted the internal thermonuclear fuels in its core at the end of its life, the core becomes unstable and gravitationally collapses inward upon itself, and the star's outer layers are blown away.

What is a gamma-ray burst?

A second theory proposes that gamma-ray bursts are the result of a merging between a neutron star and a black hole or between two black holes. ... A hypernova explosion can occur when the largest of the supermassive stars come to the end of their lives and collapse to form black holes.

How big does a star have to be to potentially form a black hole?

A star with a mass greater than 20 times the mass of our Sun may produce a black hole at the end of its life. In the normal life of a star there is a constant tug of war between gravity pulling in and pressure pushing out. Nuclear reactions in the core of the star produce enough energy to push outward.

Where in the Galaxy would you find dust, gas, open clusters or globular clusters?

All the globular clusters are very far from the Sun, and some are found at distances of 60,000 light-years or more from the main disk of the Milky Way.

How are black holes detected?

Black holes are detected as surrounding material (like gas) is funnelled by the force of gravity into a disk around the black hole. The gas molecules in the disk swirl around the black hole so fast that they heat up and emit X-rays. ... Black holes can also be detected by watching for motions of stars near the black hole.

How are they supported against gravitational collapse?

Intergalactic space is filled with clouds of gas (mostly H + He) and dust known as molecular clouds. These clouds are supported against gravitational collapse by their thermal pressure, but if the clouds get too big massive, gravity wins and they can start to collapse. This is the first step towards star formation.

How is a black hole's gravity the same as / different from a normal star's gravity?

Remember: same mass, same distance from the center. So the answer is NO! Gravity at the same distance is exactly the same, because the mass is the same.

What is meant by "escape velocity"? What is special about the event horizon?

The event horizon is the threshold around the black hole where the escape velocitysurpasses the speed of light. According to Einstein's theory of special relativity, nothing can travel faster through space than the speed of light

Why do astronomers think the Cygnus X1 system includes a black hole?

The majority of the star's mass was shed, most likely as a stellar wind. If this star had then exploded as a supernova, the resulting force would most likely have ejected the remnant from the system. Hence the star may have instead collapsed directly into a black hole.

How does time pass differently, near a black hole?

To a distant observer, clocks near a black hole would appear to tick more slowly than those further away from the black hole. Due to this effect, known as gravitational timedilation, an object falling into a black hole appears to slow as it approaches the event horizon, taking an infinite time to reach it.

What is gravitational redshift?

light loses energy as it escapes from a source of gravity result: longer wavelengths

What are the components of our galaxy?

structure can be viewed as consisting of six separate parts: (1) a nucleus, (2) a central bulge, (3) a disk (both a thin and a thick disk), (4) spiral arms, (5) a spherical component, and (6) a massive halo. Some of these components blend into each other. Three views of the Milky Way Galaxy.