Space

Question 1

What is intensity?

Answer 1

Intensity is the rate of energy (power) per unit area (per metre) at right angles to the direction of travel of the wave.

Symbol “I” measured in Wm^-2.

Question 2

What is luminosity?

Answer 2

Luminosity is the rate at which energy of all types is radiated by an object in all directions.

Symbol “L” measured in W.

Question 3

How do you calculate intensity if you know luminosity?

Answer 3

I= L/4πd2.

This is an equation that can be used on Earth where the intensity I, of the radiation of the star reaches us. As the radiation of the star spreads out, it becomes more diluted, therefore intensity I and distance d obey the inverse square law.

Question 4

What is a red giant?

Answer 4

It is a large star, somewhat cooler than our Sun, e.g. 3000K.

Question 5

What is a white dwarf?

Answer 5

It is a small, hot star, perhaps 10000K.

Question 5

What is a blue supergiant?

Answer 5

It is a very large star, very hot star, perhaps 25000K.

Question 6

What is Stefan-Boltzmann law?

Answer 6

It states that the power output from a black body is proportional to its surface area and the fourth power of its temperature in kelvin (K)

L = δAT4. Δ = 5.67 x 10-8 Wm-2K-4.

Question 7

What is Wien’s law?

Answer 7

It is the relationship between the peak output wavelength and the temperature for a black body radiator.

The equation is: λmaxT = 2.898 x 10-3 mK.

Question 8

What is a standard candle?

Answer 8

A standard candle is a stellar object (i.e. a star) that has a known luminosity.

Question 9

How would you calculate astronomical distances using measurements of intensity from standard candles?

Answer 9

The standard candle’s intensity on Earth is measured. As you know the luminosity of the standard candle, you can use the inverse square law.

d2= L/I 4π

Question 10

How would you calculate astronomical distance to a nearby star using trigonometric parallax?

Answer 10

The nearby star is viewed from two positions at 6 month intervals. The change in angle of the star against a backdrop of fixed stars is measured. Then trigonometry is used to calculate the distance to the star.

Question 11

What is an astronomical unit AU?

Answer 11

The mean distance between the Earth and the Sun.

Question 12

What is one light year?

Answer 12

This is the distance travelled by electromagnetic waves in one year

Measured in ly (light year) and approx. equivalent to 63000 AU

Question 13

What is one parsec?

Answer 13

A star is exactly one parsec away if the angle of parallax, θ = 1 arcsecond. The parsec is a distance unit and is 3.09 x 1016 m.

Question 13

What is one arcsecond?

Answer 13

1 arcsecond = 1/3600 degree

Question 14

What does the evolution of a star depend on?

Answer 14

Its mass.

Question 15

What is the life cycle of a star for a small mass?

Answer 15

All starts start as a cloud of dust and gas (nebula) and they come together due to the force of gravity until it forms a protostar. It continues to accumulate dust and gas until it increases in density and temperature

Temperature becomes high enough for nuclear fusion of hydrogen nuclei to occur; hydrogen nuclei fuse to form helium releasing a large amount of energy

Very high temperatures are needed to make sure that the hydrogen nuclei have enough kinetic energy to overcome the repulsive forces and fuse the nuclei together

Now the outwards forces due to the radiation pressure of nuclear fusion equal the inward forces due to gravitational attraction therefor forces are balanced. This a main sequence star (like our Sun).

Main sequence is a long-lived phase where the star runs out of hydrogen so nuclear fusion stops. The outward forces due to radiation pressure stop, so there is a resultant inward force due to gravity. The helium core contracts and as it contracts, the temperature increases due to the mass/weight of the star. Outer layers expand and cool and the star becomes a red giant

The helium core continues to contract until the temperature and density of the helium core becomes high enough for it to start to fuse helium nuclei. The outer layers become more unstable so the star ejects its outer layers into space as a planetary nebula. The core left behind becomes a very dense and hot solid called a white dwarf. No further fusion reactions occur.