Classification of stars

Question 1

define luminosity of a star

Answer 1

The luminosity, L, of a star is the total amount of electromagnetic radiation it
emits each second, measured in Joules per second or watts. (It is the power of the
star). (not brightness)

(It depends on the absolute temperature of the star, T, in kelvins; the higher the
temperature, the greater the luminosity.
It also depends on the surface area of the star, A in square metres; the greater the
value of A, the greater the luminosity.)

Question 2

define the intensity of a star

Answer 2

The intensity of radiation hitting Earth from a star, I, in watts per meter square
is defined as the energy each second from the star hitting 1 square meter on Earth
at right angles.

Question 3

define apparent magnitude of a star

Answer 3

The apparent magnitude, m of a star is a measurement of how bright the
star appears to us on Earth. It is a subjective measurement in that it depends on
how far away the observer is from the star- the apparent magnitude would change at
different distances.

Question 4

history of apparent magnitude

Answer 4

classified the stars according to their brightness- the brightest stars were given
apparent magnitude 1 and the dimmest stars that could be seen by the naked eye
were given apparent magnitude 6 with other stars being classified in between.
In the 1800s, the scale was changed to a logarithmic scale where a magnitude 1
star had a brightness of 100 times a magnitude 6 star.

Question 5

what is the difference between two stars one magnitude apart

Answer 5

A difference between two stars one magnitude apart is a factor of 100^1/5 or
2.51 [ 2.515 = 100].

Number of times that one star is brighter than another
= 2.51 ^(difference in apparent magnitude)

Question 6

define light year

Answer 6

the distance light travels in a year

Question 7

define astronomical unit

Answer 7

The astronomical unit (A.U.) is the mean distance from the Sun to the Earth

Question 8

what is an arc second ?

Answer 8

1 arc second is 1/3600 of a degree. One second is 1/60 of a degree.
1 second = 60 arc seconds.

Question 9

define one parsec

Answer 9

One parsec (pc) is defined as the distance to a point in space where the
radius of the Earth’s orbit of the Sun subtends an angle of 1 arc second. This means
that the parallax angle is 1 arc second. A diagram may help to explain it.

Question 10

define absolute magnitude

Answer 10

The Absolute magnitude of a star or galaxy is what the apparent magnitude
would be if the star/galaxy was 10 parsecs away from Earth.

Question 11

What is the absolute magnitude of the sun

Answer 11

The absolute

magnitude of the Sun is +4.77 (Learn the value).

Question 12

What are standard candles

Answer 12

Objects in the sky whose absolute magnitude are known (such as Cepheids)
are called standard candles

Question 13

What is a black body

Answer 13

A black body completely absorbs all wavelengths of radiation falling on it and
can emit all wavelengths.
We assume that stars are black bodies and so they obey Wein’s Law.

Question 14

shape of a black body radiation curve

Answer 14

You need to know the Black Body radiation curve for different temperatures.
Notice that he curve is steeper on the short wavelength side and that the graph
tends to zero on the right hand side.

Question 15

what is wein’s law

Answer 15

(wavelength)max T = constant = 2.9 × 10-3 mK
The link for a black body between surface temperature and the wavelength at
which peak power is emitted (max)

Question 16

What is Stefan law

Answer 16

The total Luminosity, P (for power), in watts, from a
spherical black body is given by
P = A(theta)T
4
where (theta) is Stefan’s constant which is 5.67 × 10^-8 Wm^-2 K^-4
For a sphere, A = 4πR2

Question 17

define The intensity of radiation hitting Earth from a star

Answer 17

The intensity of radiation hitting Earth from a star, I, in watts per meter square
is defined as the energy each second from the star hitting 1 square meter on Earth
at right angles.

Question 18

how is The intensity, I is linked to the luminosity of a star(equasion)

Answer 18

I = L/4D² where D is the distance to the object.
The relationship I = L/4D² assumes that no radiation is absorbed or scattered by
atoms in space or in the atmosphere on its way from the star to the detector on
Earth.

Question 19

What is the classification of the sun and its temperature

Answer 19

The Sun is a G2 star with a temperature of 5800K.

Question 20

why are absorption spectra produced

Answer 20

Light from stars is emitted from the hot centre of the star and then passes
through cooler gases near to the surface of the star. Some wavelengths are
absorbed by these cooler gases causing an absorption spectrum to be seen when
the spectrum from a star or galaxy is viewed.

Question 21

where does The visible absorption spectrum in hydrogen come from

Answer 21

The visible absorption spectrum in hydrogen comes from electrons in the
n=2 level absorbing photons. This is called the Balmer absorption spectrum.
So, the number of electrons in the n=2 level depends on the temperature
of the gas and so therefore does the darkness or “intensity” of the Balmer
absorption lines. Therefore, the darkness of the lines can be used to measure the
temperature of the gas and classify it into its spectral class.

Question 22

Life cycle of stars

Answer 22

The star goes into an equilibrium state, where it will stay for many millions of
years, until the hydrogen in the core has run out. It is now called a Main Sequence
star.
Stars become Red Giants as they come to the end of their lives after they have
been on the Main Sequence.
Red Giants have high luminosity but relatively low temperature.
The core of low mass stars, like the Sun, turn into White Dwarf stars at the end
of their lives after turning into a Red Giant.
The core, which is very hot, dense and with a small diameter is left behind and
is called a White dwarf. White dwarfs are mostly in the OB class and, although they
are hot, are not bright (they have a low absolute magnitude)

Question 23

what is a supernova

Answer 23

A supernova is where a giant star explodes at the end of its life. It has a rapid
and short- lived increase in absolute magnitude.

For some very massive stars, bursts of high energy gamma rays are emitted
when they supernova. These bursts usually last minutes but can last hours.
The energy output of a type 1a supernova (when a white dwarf near to another star
explodes) is about 1044J. This is similar to the energy given out by a star like the Sun
over its whole lifetime. Other types of supernova may give out more energy.

Question 24

how dense are white dwarfs ?

Answer 24

Neutron stars are very dense- about 4 x 1017 kgm-3
This density is the density
of nuclear matter

Question 25

properties of white dwarfs

Answer 25

Neutron stars are very dense- about 4 x 1017 kgm-3
neutron stars have a very strong magnetic field
The spinning neutron star is called a PULSAR- it is a very strong radio
source.

Question 26

what is a black hole and when does it form

Answer 26

If the core of a star is 3 times the mass of the Sun, the gravitational forces are so
strong that the laws of physics predict that the star will collapse to a single point called
a singularity. The gravitational field is so strong the gravitational pull is so strong that
nothing can escape, not even light.

It will then become a BLACK HOLE, which is an object where the escape
velocity is greater than the speed of light.

Question 27

what are at the center of galaxies

Answer 27

It is thought that quasars are supermassive black holes at the centre of
galaxies which are very strong emitters of radio waves.

Question 28

What is the event horizon and therefore the Schwarzchild radius

Answer 28

The event horizon is the boundary of the black hole, where the escape
velocity is the speed of light. The distance between the centre of the black hole
and the event horizon is called the Schwarzchild radius.