ASTR 1102 HYNES TEST 1

An astronomer says that a globular cluster contains about 7x10^5 stars (seven times ten to the power five, 7e5 on a calculator). How many stars is this in non-scientific notation?

700,000

Light has a particle nature, and these particles are called photons. Which region of the electromagnetic spectrum has the lowest energy photons?

Infrared

Which of the following tools does a typical professional astronomer spend most time using?

Computers

Which of the following does NOT differ between different kind of electromagnetic radiation?

The speed at which the radiation travels

The star Betelgeuse is about 500 light years away from us in the constellation Orion. If this star underwent a supernova explosion right now, approximately how long would it be until we found out about it?

500 years

Which of the following could be part of the constellation Aquarius?

All of the other answers are correct

Compared to visible light, radio waves have

Longer wavelength and lower energy per photon

The BEST test of a scientific hypothesis is

how well it predicts new observations

Which type of electromagnetic radiation has a wavelength adjacent to, but shorter than ultraviolet light?

X-ray

Which of the following statements comparing radio and infrared radiation is INCORRECT?

Radio waves have a shorter wavelength than infrared photons.;

At what wavelength would you expect to see the peak in the spectrum of a star of temperature 2900K?

1000 nm

Wien's Law says that w=0.0029/T, where T is the temperature and w is the wavelength. So w=0.0029/2900=0.000001m=1000nm.

At what wavelength would you expect to see the peak in the spectrum of a star of temperature 29000 K?

100 nm

Wien's Law says that w=0.0029/T, where T is the temperature and w is the wavelength. So w=0.0029/29000=0.000001m=100nm.

When an astronomer calls an object a black body, what do they mean?

It absorbs all the light that falls on it

A blackbody absorbs all of the light that falls on it and reflects none. It may still be very hot, and emit radiation with a color dependent on its temperature.

A hot star has a temperature of 23,000 K (about 4x hotter than the Sun). How much more energy does it emit per square meter of its surface than the Sun?

256x more energy?

The Stefan-Boltzmann Law says that the energy emitted per square meter depends on the fourth power of the temperature. If the star is 4x hotter then it emits 4x4x4x4=256x more energy.

A hot, glowing, solid, object surrounded by cool gas emits which type of spectrum?

Absorption line

Cool gases in front of a hotter background source of light produce absorption line spectra.

The spectrum of an astronomical object contains only emission lines. What are you most likely to be seeing?

A hot, thin, transparent gas

This is based on Kirchhoff's laws. The 2nd Law says that hot, transparent gas produces an emission line spectrum.

A very cool star has a temperature of 2,900 K (2x cooler than the Sun). How much less energy does it emit per square meter of its surface than the Sun?

16x less energy

The Stefan-Boltzmann Law says that the energy emitted per square meter depends on the fourth power of the temperature. If the star is 2x cooler then it emits 2x2x2x2=16x less energy.

Which of the following objects emits a continuous spectrum in which light is present at all wavelengths?

The filament of a light bulb

Continuous spectra typically come from solid objects or opaque gases (like stars). A light bulb is a good example (we looked at this in class). Low density gases, on Earth or in space, typically radiate emission line spectra.

A hot star has a temperature of 17,000 K (about 3x hotter than the Sun). How much more energy does it emit per square meter of its surface than the Sun?

81x more energy

The Stefan-Boltzmann Law says that the energy emitted per square meter depends on the fourth power of the temperature. If the star is 3x hotter then it emits 3x3x3x3=81x more energy.

Assume the spectrum of a star is exactly a blackbody. If the surface temperature of the star HALVES, what will happen to the peak wavelength of the spectrum?

It will double in wavelength

Wien's Law is an inverse relation - higher temperature means shorter (smaller) wavelength. There are no powers, so half the temperature means double the wavelength.

Blackbody A is at 20000 degrees K while blackbody B is at 5000 degrees K. Therefore blackbody A gives off how many times more total energy than blackbody B?

256

The total energy given off depends on the fourth power of the temperature (Stefan-Boltzmann Law). If the temperature is 4x higher, the energy output is 4x4x4x4=64x higher.

Assume the spectrum of a star is exactly a blackbody. If the surface temperature of the star INCREASES by a factor of four, what will happen to the peak wavelength of the spectrum?

It will be a quarter of the previous wavelength?

Wien's Law is an inverse relation - higher temperature means shorter (smaller) wavelength. There are no powers, so four times the temperature means one quarter the wavelength.

Which of the following types of astronomical observations CAN EASILY be performed by a telescope on the ground?

Radio

The atmosphere blocks all kinds of electromagnetic radiation except radio, optical, and some infrared wavelengths. Of the options presented only radio is possible from the ground

How does the nuclear energy production in the core of the Sun compare to the energy radiated from surface of the Sun?

The core must produce the same amount of energy as is radiated from the surface.

The Sun is in thermal equilibrium which means heat is balanced - the heat leaving the surface must equal that generated in the core.

The surface layers of the Sun are very massive. What stops the Sun from collapsing over its own weight?

The pressure of the very high-temperature gas within the Sun supports the outer layers.

The layers of the Sun are supported by pressure from the layers below them. This is called hydrostatic equilibrium - pressure balancing gravity. Exactly the same thing happens when you float in water.

A star shows absorption lines redshifted from 600.0nm to 599.4nm How fast in the star moving?

300 km/s towards us (300,000 m/s)

Firstly, the lines are blue-shifted, so it is moving towards us. This immediately limits your choices. The change in wavelength is 0.6nm, or 0.001 times the wavelength. This means that the speed is 0.001 times the speed of light, or 300 km/s (300,000 m/s). In math, v/c=0.6/600, so v=0.6/600*c = 0.001c = 300,000 m/s.

How is the Sun's luminosity produced?

Nuclear fusion

Only nuclear fusion is important in producing the Sun's current luminosity. Gravitational contraction did play a role in getting the process started 5 billion years ago. Chemical burning has never been relevant to the Sun.

Which of the following correctly describes the structure of the Sun's interior from inside to outside

Core, radiative zone, convective zone

A star is moving away from the Earth at 200 km/s. The hydrogen-alpha absorption line normally has a wavelength of 656.3nm. What can you say about the wavelength at which it will appear in the spectrum of the star?

It would be more than 656.3 nm

The star is moving away from us so its spectrum is red-shifted. Red-shifts correspond to longer wavelengths, so the wavelength of the line would become longer than 656.3nm.

How was the chemical composition of the Sun 3 billion years ago different from today?

It had more hydrogen

The Sun generates energy by turning hydrogen into helium by thermonuclear fusion. That means that 3 billion years ago it must have had more hydrogen and less helium than it has today. Carbon and iron have not significantly changed

One of the processes carrying energy from the core of the Sun to its surface is convection. What does the process of conviction involve?

The motion of hot gases

In convection, circulating currents of hot gas carry heat upwards. On a day to day basis, we know convection as 'Hot air rises'

The Sun contains

74 percent hydrogen

The Sun's composition is 74 perfect hydrogen, 25 percent helium, and 1 percent other elements.

Which of the following best describes all the processes important in energy getting out of the Sun?

Radiation and convection

Energy from the core first escapes by radiation through the radiative zone, then by convection through the convective zone, and ultimately leaves the surface as radiation

An object is moving away from us at a great speed. Therefore, the light it emits is Doppler shifted to:

Redder, longer wavelengths

Receding objects become redder because their light is stretched to longer wavelengths.

How does deuterium differ from hydrogen?

Deuterium has a different number of neutrons to hydrogen.

Deuterium is a heavier isotope of hydrogen containing the same number of protons (one), but one neutron where normal hydrogen has no neutrons. Its mass is about double that of hydrogen.

What can you learn about a star from its spectrum?

Its velocity towards or away from us, its chemical composition, and its temperature.

While we have not studied stars in detail yet, we have discussed the fact that temperatures, chemical composition and velocity can all be studied with the spectrum

The Sun's lifetime is about 10 billion years. If another star had the same mass as the Sun but was ten times LESS luminous, how long would it be expected to live?

100 billion years

If the star had the same mass as the Sun it would have the same amount of fuel. If it is ten times less luminous, then ten times less energy must be produced in the core and so it is burning its fuel ten times slower. If it is burning fuel ten times slower, then it will last ten times longer, or 100 billion years.