# Astronomy 13-14

## 20 terms

### What information about the sun do we need to know about our Sun to calculate its structure?

Its mass and chemical composition.

### Why is hydrogen burning the main energy source for main sequence stars? Give 2 reasons.

1. It's the most abundant element in stars. Largest fuel source available to main sequence stars.

2. Hydrogen fusion converts a larger fraction of mass to energy than any other fusion process. Most efficient.

### What are 3 ways in which energy could be transported within a star? Explain.

2. Convective = motions of molecules/atoms from one place to another

3. Conduction = transference of energy from one set of molecules/atoms to another

### Through hydrogen burning, the Sun loses approx 4 million tons/sec. If it burns hydrogen at this rate for 10 billion years, what % of its original mass will it lose in all? Sun's mass = 1.99 x 10*30 kg 1 ton = 1,000 kg

4 billion (9 zeros) x 602 x 24 x 365 x 10 billion (10 zeros) = 1.26 x 1027 kg (total mass lost)

mass lost/total mass = 1.99 x 10*30 = 0.063%

1.26 x 10*27
0.063%

### Calculate the amount of energy released by converting 4 hydrogen atoms into 1 helium atom. Hydrogen atom mass = 1.67 x 10*-24 Helium atom mass = 6.65 x 10*-24 Speed of light = 3 x 10*8

4Mh-Mhe = 4(hydrogen atom mass) - (helium atom mass) = 3 x 10*-26

E=mc*2
mass x 1/1000 x (speed of light) = 2.7 x 10*-12 J

3 x 10*-26 g
2.7 x 10*-12 J

### What is the main property of a star that determines all its other properties?

The star's mass has the most effect on all other properties.

### How are the following properties organized along the main sequence of an H-R diagram: luminosity, temp., radius, mass?

Upper left = more luminous, hot, large, more massive stars

Lower right = low-luminous, cool, small, low-mass stars

### Explain how we can use spectroscopic parallax to determine the distance to a star farther away than a few hundred light years.

Find the temperature using its absorption spectrum line. If it is a main sequence star and you know the temperature then you can read the luminosity from the diagram. Then measure brightness and use the Inverse square law of radiation to find the distance.

### Star A emits a peak wavelength of 500nm; star B emits its peak energy at a wavelength of 750nm. If both stars are the same size, which star is hotter and by how much?

Use Wiens Law. (peak wavelength of A)/(peak wavelength of B) = 500/750 = 2/3

Find inverse: 1/2/3 = 3/2 = 1.5

Star A is 1.5 times hotter than Star B

### You observe a binary star system and find that star X has a velocity of 35 m/s. What is the ratio of the two masses of the two stars (Mx/My).

Mx/My = Vy/Vx = 35/10 = 3.5

Star X is 3.5 times as massive as star Y.

### Star A is exactly the same color as Star B and appears equally bright. Through stellar parallax we find that Star B is twice as far away from us as Star A. Determine which star is bigger and how much bigger it is.

If Star B is twice as far away, it must be 4 times as luminous as Star A. If Star B is 4 times as luminous, it must be twice as big as Star A.

Star B x2

### Explain how astronomers can use the blue and visual filters to determine the temperatures of stars.

They compare the relative intensities of measured through each filter. Stars with more blue light than visual light are hotter, whereas stars with more visual light than blue light are cooler.

### What property of stars allows us to use Stefan's Law and Wien's Law to understand the radiation they emit?

Because stars are fairly dense their radiation behaves according to the laws of Planck radiation.

### A star with a parallax of 0.025 arcseconds is at a distance of how many parsecs?

1/0.025 arcseconds = 40 parsecs

### How was the unit of length known as a parsec derived?

An object at a distance of 1 parsec has a parallax exactly equal to 1 arcsecond.

### In the text we considered a case of a "too-large" sun. Show that a star with the same mass, composition, radius, and luminosity as the Sun, also leads to a contradiction.

A higher temperature means more energy produced in the core. This is because the proton-proton chain runs faster at higher temperatures.

So to have an equilibrium, the center's gravity must be larger, so the matter must be denser (we must increase mass or decrease radius).

More energy production will lead to a higher luminosity on the surface unless radius is decreased.

The only way to keep mass and luminosity constant is for the radius to both increase and decrease simultaneously.

### What is limb darkening? Explain why limb darkening occurs in the Sun.

Limb darkening means that the edges of the Sun are fainter than the center. It occurs because at the edges, we're not looking as deeply down into the interior of the Sun as we are the center.

### Explain why magnetic fields trap coronal gas over much of the solar surface but allow it to escape in coronal holes.

Most of the solar magnetic field consists of closed field loops. Coronal material can move along but not cross these loops. This traps the material.

Coronal holes are regions where field lines are open and stream away from the solar surface. Coronal material can follow these lines out through the solar system.

### How do long periods of strong solar activity effect near-Earth orbiting spacecraft.

At solar maxima, the amount of extreme ultraviolet and X-Ray radiation that reach Earth increases significantly. This causes extra heating in the upper atmosphere, which responds by swelling. Swelling increases drag on near-Earth satellites and can make their orbits decay.

### When during its 11-year cycle is the Sun the most luminous? What might this have to do with Maunder minimum?

Sun is most luminous during solar maxima, when sunspots on its surface are at a max.

During the Maunder minimum few sunspots were seen on the surface which implies it was less luminous and caused Earth to be a bit colder than today.