Into the Edge of the Stars Humanity’s changing vision of the cosmos
Presenter: Haileyesus Wondwossen
Basic measurement.
How old our universe is?
Evidence that the universe had a beginning.
Size comparison.
The universe-Earth
Faster travel.
Search for life-bearing planets
Mystery question
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5. Stellar colors
• The stars show different colors
according to their temperature
• Relationship between spectral
classification, temperature and
color of stars
6. Hertzsprung-Russell
Diagram
• The stars can be represented in an
empirical diagram, using the surface
temperature (or spectral type) and its
brightness (or absolute magnitude).
• In general, the stars occupy certain
regions of the diagram.
• It is possible to know the type of star
and its evolutionary stage.
8. CHARACTERISTICS OF A STAR READY TO
EXPLODE AS A SUPERNOVA
• 10 million years burning hydrogen
inside its core (main sequence).
• 1 million years burning helium
• 300 years carbon
• 200 days the oxygen
• 2 days in consuming silicon
the explosion of the supernova is
imminent.
A star of 20 solar masses lasts
9. Sun
• Start with a normal star like the
Sun. Fusion of protons into helium
in the star’s center generates heat
and pressure that can support the
weight of the star. The Sun was
mostly made of hydrogen when it
was born, and started with
enough hydrogen to last like this
for about 10 billion years.
10. Cont..
• When it begins to run out of hydrogen in its
center, not enough heat and pressure are
generated to balance the star’s weight, so
the core of the star gradually begins to
collapse.
• As the core collapses it gets hotter, though
no extra heat has been generated, just
because it compresses
• It gets so hot that light from the core causes
the outer parts of the star to expand and
get less dense, whereupon the star looks
cooler from the outside. The star is
becoming a red giant.
11. • Eventually the core gets so hot
that it is possible for helium to
fuse into carbon and oxygen.
Extra heat and pressure are once
again generated and the core
stops collapsing; it is stable until
the helium runs out, which takes
a few million years. The outer
parts of the star aren’t very
stable, though.
12. Cont..
• Eventually the core is all carbon and
oxygen, no additional heat and gas
pressure is generated, and the core
begins collapsing again. This time
the density is so large the electrons
so close together that electron
degeneracy pressure begins to
increase significantly as the collapse
proceeds.
13. Cont..
Electron degeneracy pressure eventually
brings the collapse of the core to a halt,
before it gets hot enough to fuse carbon
and oxygen into magnesium and silicon.
The unstable outer parts of the star fall
apart altogether; they are ejected and
ionized by light from the core, producing
a planetary nebula.
14. Habitable planets
• Habitable zone (HZ) = zone around the star
where liquid water can be found
• L* increases during the main sequence phase
• the habitable zone moves
• Ideal location: in the continuously habitable
zone (CHZ)
• Complication by possible greenhouse effect
• depends on the planet’s atmosphere
Habitable zone at the beginning of stars life
Habitable zone at the end of stars life
Continuously habitable zone
Star
15. Around which stars?
• O, B, A, F stars: life too short < 3 GYr
• M stars: very long life but low luminosity stars
• → (1) HZ very narrow and no CHZ (but 200 GYr not necessary)
• (2) HZ very close to the star → synchronous rotation
• → deadly radiation from stellar corona?
• G stars: good compromise
• K stars: maybe same problems as M stars
• Non binary main sequence G stars are privileged targets, the case of K and M stars in
open and under deep investigation
• → ~ 90% of stars in our Galaxy
• Exoplanet = extrasolar planet = planet orbiting a star that is not the Sun
18. FORMATION OF PLANETARY SYSTEMS
• Contraction of proto stellar nebula
• star at the center, surrounded by a disk of gas and dust
• Collisions between dust grains → aggregates
• size increases and may reach a few km: planetesimals
• Gravitation starts to play a role
• even more collisions with:
• fusion and size increase
• or destruction of aggregates
• eccentric orbits → even more collisions
Protoplanets
• The most massive planetesimals tend to grow further by capturing bodies on similar orbits
• Size ~ 1000 km → protoplanets
• The most massive can be surrounded by a disk of matter that will give birth to their satellites
• Perturbation of the orbits of small bodies by the most massive planets
• heavy bombardment and big cleaning of the planetary system
