Astronomy Exam #2

What is the chemical composition of the universe over all?

The chemical composition of the universe is hydrogen and helium that was produced by the Big Bang.

How does this compare to that of the Sun?

The Sun is mostly composed of hydrogen and helium. By mass the composition of the Sun is 75% hydrogen and 25% helium. Metals make up less than 0.1% of the mass of the Sun. The temperature of the Sun's temperature is about 10,340 Fahrenheit.

What generates the energy of the Sun? In what details, and why, is this similar to and different from other stars?

The Sun, like all stars, is able to create energy because it's essentially a massive fusion reaction. Nuclear fusion is the process that releases an incredible amount of energy in the form of light and heat. The amount of energy released by fusion in the Sun's core equals the amount of energy radiated from the Sun's surface into space. {The core of the Sun is the region that extends from the center to about 20-25% of the solar radius. It's here in the core where energy is produced by hydrogen atoms beginning to be converted into nuclei of helium.} This is possible due to extreme pressure and temperatures that exist within the core that is about 250 Billion atmospheres and 15.7 Kelvin. The core is the only part of the Sun that produces an appreciable amount of heat through fusion. 99% of the energy produced by the Sun takes place within 24% of the sun's radius. The rest of the Sun is heated by the energy that's transferred from the core through the successive lawyers eventually reaching the solar photosphere and escaping into space as sunlight or kinetic energy of particles.

The balance of what two factors controls what goes on inside the Sun?

1) Gravitational equilibrium (hydrostatic equilibrium) is between the outward push of internal gas pressure and inward pull of gravity. Gravitational equilibrium works much the same way in the Sun except the outward push against gravity comes from internal gas pressure. The Sun's internal pressure balances gravity at every point within it, keeping the Sun stable in size. Deep in the Sun's core, the pressure makes the gas hot and dense enough to sustain nuclear fusion. The energy released by fusion in turn heats the gas and maintains the pressure that keeps the Sun in balance against the inward pull of gravity.2) Energy balance between the rate at which fusion releases energy in the Sun's core and the rate at which the Sun's surface radiates this energy into space. Energy balance is important because w/out it the balance between pressure and gravity wouldn't remain steady. If the fusion in the core didn't replace the energy radiated from the surface then gravitational contraction would cause the Sun to shrink and force its core temperature to rise.

What is remarkable/unusual about our Sun compared to other stars?

Our Sun is an average sized star. There are smaller stars and larger stars, even up to 100 times larger. Many other solar systems have multiple suns, while ours just has one. Our Sun is 864,000 miles in diameter and 10,000 degrees Fahrenheit on the surface.

Why are sunspots dark?

The reason why sunspot appears dark is that the gas inside the spot where the magnetic field is strongest is only emitting about ¼ as much light as from the rest of the solar surface. If you were to rip a sunspot out from the solar surface and put it in the night sky it would appear as bright, orange gas, not a dark void. What causes sunspots? Sunspots are caused by the Sun's magnetic field welling up to the photosphere, the Sun's visible surface. They produce active regions on the Sun which often lead to solar flares and coronal mass ejections.

How long is the sunspot cycle? What effects does it have on Earth?

The duration of the sunspot cycle is around 11 years. It can also be as short as 7 years or up to 15 years.Sunspots are connected with other solar event flares and coronal mass ejections. These bubbles have strong magnetic fields and can reach Earth in a couple of days if they happen to be aimed in our direction. Once a coronal mass ejection reaches Earth, it can create a geomagnetic storm in Earth's magnetosphere. They can hamper radio communications, disrupt electrical power delivery, and damage the electronic components in orbiting satellites, shut down power, and destroy spacecraft electronics. The Sun's total output of energy is less than 0.1%. From 1645 to 1715 when solar activity ceased, when low temperatures in Europe and North America known as the Little Ice Age. It is unknown if the low solar activity caused these low temperatures or whether this was a coincidence. Other research has claimed that certain weather phenomena like drought cycles or frequencies of storms are correlated with the 11 or 22 year cycle of solar activity. Another claim is that the changes in the Sun may be responsible for Earth's recent global warming but the Sun's energy output has actually remained steady while the temperatures rise.

What is the main sequence?

The prominent streak running from the upper left to the lower right on the H-R diagram. It is the continuous and distinctive band of stars that appear on plots of stellar color versus brightness. These are the most numerous true stars in the universe that also includes the Earth's Sun.

What happens to stars when they leave the main sequence?

Eventually, a main sequence star burns through the hydrogen in its core, reaching the end of its life cycle. At this point, it leaves the main sequence. Stars smaller than a quarter the mass of the sun collapse directly into white dwarfs. White dwarfs no longer burn fusion at their center, but they still radiate heat.

What are the end states of different stars?

Life of High-Mass Star: 1) Protostar, 2) Blue-main sequence star, 3) Red Supergiant, 4) Helium core fusion supergiant, 5) Multiple shell fusion supergiant, 6) Supernova, 7) Neutron star or Black hole.Life of Low-Mass Star: 1) Protostar, 2) Yellow main-sequence star, 3) Red giant star, 4) Helium core fusion star, 5) Double shell fusion red giant, 6) Planetary nebula, 7) White dwarf.

What controls which stars end up in each end state?

The final stages in the evolution of a star depend on its mass, angular momentum, and whether it's a member of a close binary.

How do planetary nebulae connect to this?

Planetary nebuae's connect to the 6th stage for life of a low-mass star. This is where the dying star expels its outer layers in a planetary nebula leaving behind the exposed inert core. All stars begin their lives as part of nebulas. Gravity can pull some of the gas and dust in a nebula together which leads to a contracting cloud called protostar.

What controls how long a star will live?

To see how long a star lives depends on how much mass it has.

How long do you expect the Sun will live overall? How old is the Sun now?

The Sun will die out 10 Billion years from now.4,500,000,000 yrs old (2020)

What is luminosity? How does it change over a star's lifetime?

It's the total amount of power that a star emits into space.The lifetime of a star would be the amount of fuel that a star has available for fusion is directly proportional to its mass. The luminosity measures how quickly the star is using that fuel. As Mass increases, luminosity increases as well.

How do luminosity, apparent brightness, and distance connect to each other? (Understand specifics.)

Apparent brightness is a measure of power per unit area, Luminosity is a measure of power. Apparent brightness of a star obeys an inverse square law with distance. For instance, if we viewed the Sun from twice Earth's distance it would appear dimmer by a factor of 2^2=4. If we viewed it from 10 times Earth's distance it would appear 10^2=100 times dimmer. The same total amount of light must pass through each imaginary sphere surrounding the star. For instance if we focus on the light passing through the small square on the sphere located at 1 AU, we see that the same amount of light must pass through four squares of the same size on the sphere located at 2 AU. Each square on the sphere at 2 AU therefore receives only ½^2 = ¼ as much light as the square on the sphere at 1 AU. The same amount of light passes through nice squares of the same size on the sphere located at 3 AU, so each of these squares receives only ⅓^2 = 1/9. The amount of light received per unit area decreases with increasing distance by the square of the distance, making for an inverse square law. The inverse square law leads to a formula that connects Apparent brightness, Luminosity, and Distance of any light source. The formula is Apparent brightness= luminosity / 4(3.14) * distance^2. Luminosity standard of units are Watts, Apparent brightness standard units are Watts Per Square Meter. We can measure a star's apparent brightness by measuring the amount of light per square meter we receive from the star. We can then use the inverse square law to calculate a star's luminosity if we can first measure its distance or to calculate a star's distance if we somehow know its luminosity.

How do we use parallax to measure the distance to stars?

The most direct way to measure a star's distance is with stellar parallax, the small annual shifts in a star's apparent positions caused by Earth's motion around the Sun. We can calculate a star's distance if we know the precise amount of the star's annual shift due to parallax. This means measuring the angle p (parallax angle). The parallax formula is d (parsecs) = 1 / p (arcseconds).

How do we tell a star's surface temperature from looking at it?

By measuring the peak wavelength of a star the surface temperature can be determined. Astronomers classify stars according to surface temperature by assigning a spectral type determined from the spectral lines present in a star's spectrum.

Be able to list the classes (O, G, M, etc.) from hottest to coldest.

OBAFGKM. O: blue >33,000K ; B: blue white 33,000K-10,0000K ; A: white 10,000K-7,500K ; F: yellow white 7,500K-6,000K ; G: yellow 6,000K-5,200K ; K: orange 5,200K-3,700K ; M: red <3,700K. O=hottest and M=coolest.

For main sequence stars what property controls the star's temperature?


Understand the H-R diagram—what it is graphing, and what stars are in which parts of it.

The H-R diagram has a horizontal axis that represents stellar surface temperature where temperature decreases from left to right. While the vertical axis represents stellar luminosity. Each H-R diagram represents a unique combination of spectral type and luminosity. Luminosity increases leftward, stars near the upper left are hot and luminous. Stars near the upper right are cool and luminous, stars near the lower right are cool and dim and Stars near the lower left are hot and dim

How are the different elements created?

Gas between the stars composed mostly of hydrogen and helium. Hydrogen and helium were the only two elements produced in the Big Bang. The very first stars must have been born from clouds made up of only hydrogen and helium gas. Since that time, stars have transformed a small fraction of the original hydrogen. We can use spectroscopy to measure the abundances of the new elements that stars have added to the interstellar medium. Interstellar medium consists (by mass) of 70% hydrogen, 28% helium, and 2% heavier elements. Virtually all the gas between stars in the Milky Way has approximately the same chemical composition. Elements such as carbon, silicon, oxygen, and iron are often found in tiny solid grains of interstellar dust. Overall, about half the atoms of elements heavier than helium are found in dust grains. They make up about 2% of the mass of the interstellar medium and we can conclude that interstellar dust grains constitute about 1% of the molecular cloud's mass.

Understand eclipsing binaries: what their curves look like, and what we can learn from them.

Eclipsing binary is a binary star whose brightness varies periodically as the two components pass one in front of the other. They usually have two two dips where the small star passes the big star first, then the big star passes the smaller star in the binary. We can learn that the apparent brightness of an eclipsing binary system drops when either star eclipses the other.