birth and death cycle of stars explained with HR diagram

1. Birth cycle of stars
G Antilen Jacob
Research Scholar

2. Stellar dusts
• Stars form inside interstellar gas and dust known as molecular
clouds
• 10 to 20 K atom binds together to high density
• opaque to visible light.
• A protostar is a very young star that is still gathering mass from its
parent molecular cloud
• which contracts to later become a main-sequence star at the onset of
hydrogen fusion
• Protostar stars self luminous after ignition.
• Accumulate more dust due to accretion and become denser
Fc< F9 Ignition starts
Orion Nebula Home Of Stellar Cities

3. • Clumps without enough mass are too small to become stars
• They just cool and compact to become brown dwarfs orbiting in
space
• Kelvin–Helmholtz mechanism
• two bodies are separated by about 43 AU
• Gliese 229B has from 20 to 50 times the mass of Jupiter, but the
brown dwarf is compressed to the same size as Jupiter
• Stars that are 25 to 50 Ms live for only a few million years
• Jupiter is a Failed Star
A Brown Dwarf - a “Failed Star”
Located 18 ly (6 pc) from Earth in the
constellation Lepus (the Hare), Gliese 229B
was the first confirmed brown dwarf ever
observed

4. Our Star (PP chain)
• Two protons fuse together to form
a unstable 2He nucleus
10,000,000 K

5. Evolution of sun like stars
• Even while on the main sequence, the composition of a star’s core is changing
• Low-mass stars go quietly, High-mass stars go out with a bang!
• Massive stars consume their fuel very rapidly and are short-lived.
• Very low mass stars, with masses below 0.5 M☉
• Low mass stars with a mass between 0.5 M☉ and 1.8–2.5 M☉
• Intermediate-mass stars, between 1.8–2.5 M☉ and 5–10 M☉
• Massive stars generally have a minimum mass of 7–10 M☉
Morgan-Keenan (MK) classification
Oh Be A Fine Girl, Kiss Me
Easy way to
remember

6. Apparent and absolute magnitude
• mv – Mv = – 5 + 5 log10(d)
• Brightness, color, luminosity and magnitude, temperature.
• α-Centauri has 0.01 and β-Centauri has 0.61
• α 4.2ly, β 390ly. Absolute magnitude of α and β are 4.38 and
-4.53 respectively.
• Color Temperature 3000K to 50,000 K based on spectral
class

7. Hertzsprung–Russell diagram
• It’s a scatter plot of stars
• Another prominent feature is the Hertzsprung gap
located in the region between A5 and G0 spectral type
and +1 and −3 absolute magnitudes
• main sequence line, they are fusing hydrogen in their
cores
• The next concentration of stars is on the horizontal
branch helium fusion in the core
• The H-R diagram can be used by scientists to roughly
measure how far away a star cluster or galaxy is from
Earth

8. Protostar
• IF the protostar has enough temperature and luminosity to
make it onto the H-R scale
• Its mass determines where it jumps on.
HOPS 383, a young protostar in the Orion star-formation complex

9. Red Giant
• A red giant is a star that has
exhausted the supply of hydrogen
in its core and has begun
thermonuclear fusion of hydrogen
in a shell surrounding the core
• Leaving the Main Sequence
• It is cooler but bigger, so it’s
brighter
Triple alpha process
actual Hubble images of a very unstable red giant star as it emits a burst of light,

10. White Dwarf
• Electron degeneracy pressure supports the star
• No further nuclear reactions take place
• maximum mass of a stable white dwarf star.
• The currently accepted value of the Chandrasekhar limit is
about 1.4 M ☉
• Red giant branch
• Horizontal branch
• asymptotic giant branch

11. Red Super Giants
• In MASSIVE stars, when the helium is fusing, temps increase,
it expands, and becomes a SUPER RED GIANT and cools
• Gravity contracts the core until its heated enough to begin
burning the next element, CARBON.
• This process continues through the fusing of oxygen, neon,
nickel, and silicon
• as their core elements keep fusing until they reach iron
• When the core becomes iron fusion ends
600 million K
1 billion K
1.5 billion K
2.7 billion K
Ref:- mn.catholic.edu.au

12. Degenerate matter
The pressure of a fully degenerate gas
Fermi gas is a state of matter ,having ensemble of many non-interacting fermions, particles with half-integer spin
Internal Energy U0 related with fermi energy as
𝑈0
𝑁
=
3
5
𝐸𝑓
From caloric equation of state U =
3
2
PV
Zero point pressure P0 =
2
5
𝑁𝐸𝑓,
𝑃 =
(3𝜋2)
2
3ℏ2
5𝑚
(
𝑁
𝑉
)5 3

13. Planetary nebula

14. NUCLEOSYNTHESIS
• slow neutron capture reaction that occurs in the shells of
AGB stars
• The s-process involves the capture of neutrons by existing
nuclei (Fe-56) to form heavier ones
• rapid neutron capture reaction, also called the r-process.
• neutron flux to form highly unstable neutron rich nuclei
which very rapidly undergo beta decay to form more stable
nuclei with higher atomic number and the same atomic
mass.
• The neutron density is extremely high, about 1022-1024
neutrons per cubic centimeter.

15. s-process
• Red giant nucleosynthesis also produces some free
neutrons in reactions such as:
• 13C + 4He –> 16O + n
• 22Ne + 4He –> 25Mg + n
• These neutrons can then be captured by other nuclei
in the slow neutron capture (s) process.
• Because the neutron flux is low, a capture will
occur only every few decades, so that gaps in
nuclear stability cannot be bridged as the newly
created unstable isotope will decay before a second
neutron is captured.
Wikipedia

16. The e-process
• As the finale approaches, the star has become a cosmic onion of sorts, with layers
of heavier and heavier elements.
• A new core consisting mainly of 28Si has been created.
• At temperatures near 109 K and densities above 107 g/cc a process known as
silicon burning, or the e-process (for equilibrium).
• This process is really a variety of reactions that can be summarized as the
photonuclear rearrangement of a gas originally consisting of 28Si nuclei into
one which consists mainly of 56Ni, which then decays with a half-life of 6
days to 56Fe, the most stable of all nuclei.
• The e-process includes reactions such as:
28Si + γ ⇄ 24Ne + 4He
28Si + 4He ⇄ 32S + γ
32S + 4He ⇄ 36Ar + γ
• While these reactions can proceed in either direction, there is a tendency for the
build-up of heavier nuclei with masses 32, 36, 40, 44, 48, 52, and 56, Partly as a
result of the e-process, these nuclei are unusually abundant in nature. A variety of
minor nuclei are produced as well.
• This continues for a few days at most. Finally, the inner core has been converted
completely to 56Ni and 56Fe, the latter the most stable of all nuclei. Exogenic
fusion reactions are no longer possible.

17. Supernovae
• Supernovae can have several causes. We are
mainly interested in Type II.
• Most of the energy released in a supernova is
carried away by neutrinos.
• Something like 10% of the star’s mass is converted
to energy.
• A supernova produces enough light to outshine an
entire galaxy for weeks or months.
Galaxy NGC 2770 NASA image

18. How to calculate the age of stars?
• Globular(1000 – 1M) and Galactic clusters( 1-1000)
• Steller population
• The cluster’s age is equal to the age of the main-sequence
stars at the turnoff point

19. The Evolution of Binary-Star Systems
• If the stars in a binary-star system are
relatively widely separated, their
evolution proceeds follows normal path,
If they are closer it will lead to unusual
evolutionary paths

20. Thank you