Intro to Stars

Question 1

What was the elemental composition of the universe immediately following the big bang

Answer 1

75% 1H, 24% 4He, with traces of 2D, 3He, and Li

Question 2

what is the heaviest element that can form by nuclear fusion reactions in stars

Answer 2

Iron, Fe

Question 3

how are the heaviest elements formed

Answer 3

by neutron capture in stars or during supernova (violent explosions of stars)

Question 4

what are the axis on a Hertzsprung-Russel (H-R) diagram, and what is a H-R diagram used for? What can the x axis be replaced for?

Answer 4

H-R diagram is used to classify stars into different groups. It is a plot of stellar luminosity, y axis, versus surface temperature (of the star), x axis. Often, the x axis (surface temperature) of a star is replaced by its spectral type.

Question 5

what is the luminosity (L) of a star defined as?

Answer 5

the total amount of energy that a star radiates in 1 second

Question 6

what is the equation for the relationship between a stars luminosity, temperature and radius? what can we do to normalise it to the Sun?

Answer 6

L = R^2 x T^4. we can divide this eqn by the L, R and T of the sun to get an L/Lsun ratio.

Question 7

how is the spectral type of a star determined?

Answer 7

by looking at the strength of the hydrogen absorption lines of a star in the visible part of the spectrum (~400-800nm).

Question 8

why do we classify stars, and why do we need to have a H-R diagram?

Answer 8

H-R diagram allows you to predict how far away a star is by comparing apparent magnitude of stars with absolute magnitudes of known stars. but more importantly, the H-R diagram allows us to predict the life cycle of stars.

Question 9

what percentage of stars plot on the main sequence?

Answer 9

90%

Question 10

where do giants and supergiants plot on H-R diagram relative to the Sun

Answer 10

giants and supergiants plot in the top right. they can be much colder, but are 10-100 (giant) or 10-1000x (supergiant) larger than the sun and much more luminous.

Question 11

where do white dwarfs plot on H-R diagram

Answer 11

on the bottom left, they are very hot but small so not luminous. not on the main sequence

Question 12

where do red dwarfs plot on H-R diagram

Answer 12

on the lower end of the main sequence stars, small and cool therefore not very luminous

Question 13

when does star formation begin

Answer 13

when fragments of giant molecular clouds (low density clouds of gas and dust) contract to form dense cores which evolve into protostars

Question 14

what external triggers can cause collapse of a giant molecular cloud?

Answer 14

a shockwave - produced by e.g. supernova explosion, collision of molecular clouds, collision of galaxies

Question 15

why are protostars difficult to detect

Answer 15

they are hidden in an envelope of cold dusty gas (cocoon)

Question 16

how does a protostar form

Answer 16

when the density in a fragment of a molecular cloud is high enough it will start to contract. as collapse of the cloud fragment continues it develops a region of high density in the centre and a low density envelope. mass continues to spiral inwards, heating up as they gather speed, and heating up the core into a protostar.

Question 17

what radiation do protostars emit

Answer 17

infrared

Question 18

how does a protostellar disk form

Answer 18

contraction of the giant molecular cloud fragment continues leading to the flattening of the cloud which develops into a protostellar disk that spins around a central protostar.

Question 19

when does a protostar become a main sequence star?

Answer 19

when it becomes massive enough to reach the mains sequence and begin nuclear fusion

Question 20

what is the birthline? when are stars on the birthline

Answer 20

once the protostar is hot and energetic enough it blows away its shroud of gas and dust (cocoon) through stellar winds such that it is visible. on a H-R diagram the star is now between the main sequence and birthline. prior to this point they were protostars.

Question 21

what aspect of a star controls how fast it moves from the birthline to the main sequence?

Answer 21

its mass - massive stars join the main sequence much faster than small stars

Question 22

what are EGG’s?

Answer 22

evaporating gaseous globule. they are exposed regions of moleclar clouds with dense cores and may be the most likely positions for star formation

Question 23

what is the metallicity of a star? which elements count as metals to astronomers?

Answer 23

the abundance of elements present in an object that are heavier than hydrogen and helium. astronomers assume all elements heavier than He are metals.

Question 24

how does metallicity relate to a change in the composition of the universe through time?

Answer 24

the first stars that formed ~100Ma after the Big Bang were composed primarily of H and He. as successive generations of stars produce elements heavier than He, the composition of the universe changed through time. studying different stars metallicity allows insight into the change in the solar system composition.