Stellar Evolution

Question 1

Describe the use of the Messier and New General Catalogues

Answer 1

The messier catalogue was published by Messier in 1781 containing 105 fuzzy astronomical objects including nebulae, star clusters, and galaxies. They are identified by M followed by a number, M1, M2 etc.
The New General Catalogue is an extensive list containing 7840 objects. They are identified by NGC followed by a number (e.g.) NGC 287

Question 2

How to use the Bayer System for classification

Answer 2

The Greek letters denote descending brightness for stars in a certain constellation denoted by it’s shorthand
E.g. Alpha Psc is the brightest star in Pisces, Beta Leo is the second brightest in Leo.

Question 3

What is the interaction between radiation pressure and gravity in a main sequence star?

Answer 3

The energy radiated by the core (radiation pressure) balances the gravity of the star

Question 4

What is the balance between electron pressure and gravity in a white dwarf star?

Answer 4

Electron pressure - caused by electrons not being able to be squeezed together - is a new outward force, balancing gravity

Question 5

What is the balance between neutron pressure and gravity in a neutron star?

Answer 5

Neutron pressure occurs when all electrons and protons have been squeezed into neutrons. In a neutron star neutron pressure is balanced with gravity.

Question 6

What is the Chandrasekhar limit and what effect does it have?

Answer 6

Low mass stars have cores smaller than the Chandrasekhar Limit of 1.4 x the size of our sun. Massive stars have cores above the Chandrasekhar Limit, and they will each end their lives differently.

Question 7

What is the evolution of low mass stars (like the sun)?

Answer 7

1. Born from the collapse of dense clouds of gas (nebulae). This will clump into small protostars that form the cores. As the protostars collapse, the temperature increases until it is hit enough for nuclear fusion
2. It then spends 90% of its life using up hydrogen fuel as a main sequence star.
3. When the hydrogen runs out, it is mass dependent. In Low mass stars, the radiation pressure is zero, so the core collapses, increasing temperature. This then means hydrogen can fuse in the area surrounding the core. As the helium core contracts further outer layers of hydrogen expand, forming a red giant
4. The outer layers of the star continue to expand as a sphere of gas called a planetary nebula
5. The stars core stops collapsing when electron pressure balances gravity, forming a small but hot white dwarf
6. This eventually cools into a black dwarf

Question 8

What is the evolution of massive stars?

Answer 8

Born from the collapse of dense clouds of gas (nebulae). This will clump into small protostars that form the cores. As the protostars collapse, the temperature increases until it is hit enough for nuclear fusion
2. It then spends 90% of its life using up hydrogen fuel as a main sequence star.
3. When the hydrogen runs out it is mass dependent. When radiation pressure balances gravity eclmes zero, the temperature becomes high enough to fuse Helium into Carbon into Neon. This cycle of fuel exhaustion to core collapse to new fusion continues until Iron is formed in the core.. This is when the star expands into a blue or red supergiant.
4. Iron is the last element made as it cannot be fused, so the star collapses with no nuclear reactions If the star’s core’s mass is about 3 times the Sun’s mass, it is halted by neutron pressure. The star shrinks into a 20km neutron star.
5. This sudden halt of contraction causes a shock wave, and the star is blown apart in a supernova
6. If the core’s mass is greater than 3 times the Sun’s mass, even the neutron pressure can’t halt the collapse of the core, and a black hole if formed

Question 9

How do astronomers study black holes

Answer 9

1.The detection of gravitational waves is a new way to study black holes
2. The increased study of stellar evolution also contributes to our detection and understanding of black holes

Question 10

What is the evidence for black holes?

Answer 10

1. The accretion disc - a ring of dust and gas spiralling a black hole. These emit X-rays that we can detect. Allowed us to identify Sagittarius A, at the centre of our galaxy
2. Star Movement - if we a star orbiting an invisible object, it could be a black hole. This is useful for detecting black holes in binary star systems