galaxies Flashcards
What is a galaxy?
An enormous collection of stars held
together by their common gravity.
some details on galaxies
Galaxies have a wide range of masses: from 100 m stars (dwarf galaxies) to >1 t stars (giant galaxies). Lower mass galaxies are more common. Galaxies have a wide range of ages, stellar populations (the mix of stars in a galaxy), and gas content.
Anatomy of the Milky Way
The Milky Way (like any spiral galaxy) consists of:
A central bulge
A disk: extremely wide, but very thin
Disk diameter: 100,000 l.y. = 100 kl.y. (30 kpc )
Disk thickness: 1,000 l.y. = 1 kl.y. (300 pc) A large halo which surrounds almost completely the entire disk
how many stars in milky way
~100 b stars (up to 1 t )
where is milky located in the galaxy
Our solar system is located in the disk, 28,000 light years (=28 kl.y.) from centre Being situated in the plane of the disk we cannot observe clearly the structure of the Milky Way. Moreover, dusty gas clouds obscure our view because they absorb visible light. This is the interstellar medium that makes new star systems.
arms
Previously, our galaxy was thought to have 4 major arms.
In 2008, it was announced that IR images from NASA’s Spitzer Space Telescope have shown that the Milky Way’s elegant spiral structure is dominated by just 2 major arms wrapping off the ends of a central bar. It is orbited by a few small and very small galaxies (among them the Magellanic clouds are the most representative)
Contents of the Milky Way
disk
bulge
halo
Disk:
Filled with interstellar gas & dust ≡ interstellar medium (ISM), made up of H gas (atomic & molecular) & dust Younger stars Open clusters
Bulge:
Filled with denser gas & dust Young & older stars @ high density A few globular clusters may also be present here
halo
No gas/dust, or very rarified (tenuous) Old stars Globular clusters
Milky Way = typical . . .
all spiral galaxies have the
same structure
small galaxy companions
Milky Way’s strong gravity influences smaller galaxies
in its vicinity
small galaxy companions, The most important
the Large Magellanic Cloud (LMC), and the Small Magellanic Cloud (SMC), Both are irregular galaxies and much smaller than the Milky Way
Other small galaxy companions:
The Sagittarius Dwarf Elliptical Galaxy (SagDEG) → a small elliptical galaxy Canis Major Dwarf Galaxy: the closest known satellite galaxy but well hidden behind banks of dust in the plane of the Milky Way
Our galaxy’s tidal forces will ultimately
rip apart & ‘cannibalize’ these 2 small companions in ~1 b yrs
Stars in disk
- relatively young.
Plenty of high- & low-mass stars, blue & red Fraction of heavy elements same as or greater than the Sun
Stars in halo are old
→
formed early in Milky Way’s history
Mostly low mass, red stars
Fraction of heavy elements much less than the Sun → Formed when few heavy elements existed
Star formation stopped long ago when all gas flattened into disk No (or extremely little) ISM in the halo The fact that the stars in the disk have very different origins than those in the bulge & halo is also reflected in great differences between their galactic orbits .
All stars in the disk orbit around the galactic centre.
Circular motion of all stars in disk orbits → arises from gravitational attraction towards galactic centre
It is always in the same direction & in the same plane (roughly)
There is “bobbing” up & down due to the localized disk gravity → defines disk thickness
All stars in bulge &
halo also orbit around
the galactic centre, but…
They have random orientations: different directions (even opposite!) at various inclinations to disk Higher velocities & orbits are elliptical, sometimes at great distances from center
Orbital motions of mass in spiral arms of galaxies indicate that most of their mass is
NOT concentrated near the galactic centre, but the opposite is true . If most mass were concentrated in the center, the stars closer to the centre would orbit very fast, those more distant would orbit slower Measurements indicate that orbital speeds remain constant even up to great distances from the centre
Most of the galaxy’s mass resides far from the center
is distributed throughout the halo
There is little gas or dust, very few stars and no emitted light from the halo → Most of a galaxy’s mass is due to the presence of dark matter
it has a large mass-to-luminosity (M/L) ratio
Galactic recycling is essential for the formation of stars
.
Recycles material from old stars into new ones: the birth of the Sun & the Solar System could not have occurred without it. Takes place within the disk of the galaxy & its ISM
Galactic recycling gradually changes the chemical composition of
the ISM.
With each cycle more heavy elements are made by fusion in stars
This is the process due to which all heavier elements than H & He have been produced → after 10 b years of recycling, the chemical composition of the ISM is: 70%H2 , 28%He & 2% heavier elements. Different galactic regions change composition at different rates
How does it take place, and how these chemical riches
produced by stars remained in our galaxy?
It is a complex and long process, taking place in several stages
hot bubbles atomic hydrogen clouds molecular clouds star formation stellar burning/heavy-element formation supernovae and stellar winds
All stars return much of their material into ISM in 2 ways:
Through stellar winds, and Death events.
Low mass stars return most of their material into ISM via:
Gas ejection through mild stellar winds, and Mass loss through planetary nebulae
High mass stars return most of their material into ISM via:
Massive gas ejection through strong stellar winds,
Mass loss through supernovae
Heavy elements forged in (large) stars are thus returned in the ISM & contribute to formation of new stars, planets & appearance of Life
Consequently, low mass stars have much
less effect
on the ISM than high mass stars which influence & contribute to it much more significantly
Supernovae eject high-speed gas in a shock wave
Sweeps up ISM
Bubble of hot gas ( > 1 m K) excavated
Gas is strongly compressed, heated, ionized & excited → emits mainly X-rays, but also visible & IR radiation as it expands & cools
Some e– accelerated to nearly speed of light as they interact with the shock wave As these fast e– spiral around mgn. field lines threading the supernova remnant (SNR) → they emit radio waves (this radio emission is sometimes called synchrotron radiation )
Supernovae also generate cosmic rays
highly energetic particles, mainly protons (p +) & atomic nuclei -including some heavy ones- and a few e–, travelling at close to the speed of light
Our solar system is also in a huge
supernova bubble created long
time ago!
Bubbles fill
20~50% of Milky Way’s disk
Multiple supernovae can create huge bubbles of hot gas
which blow out of the galactic disk.
In many places in our galactic disk elongated superbubbles
are observed
Multiple supernovae can create huge bubbles of
hot gas part 2
The hot gas has very low densities and temperatures as high as a few m K, hence it is hot enough to emit Xrays → can be observed
The hot gas breaks out of the disk a blowout like a volcanic eruption
Part of the disk’s own material is also ejected
Hot gas bubbles/gas clouds cool
in the halo
Energy shared with swept-up ISM matter & also radiated from all shocked gas Also lose angular momentum
Only gravity slows down gradually the bubble’s expansion & eventually reverses the rise of the gas from the blowout → The galactic fountain model
Cooled gas clouds rain back down onto the disk → merge & impact the ISM in a large region of the disk
These collisions may trigger future star formation .
Heavy elements also returned & merge into disk’s ISM
What happens As ISM cools
p + recombine with e– neutral atomic H formed (neutral because is cool enough)
Atomic H emits at λ = 21 cm
Radio emission line emitted when spin state of e– flips Used to map atomic H distribution in disk with radio telescopes
Milky Way has 5 b MSun of atomic H.
Large, tenuous, 10,000 K warm clouds (1 atom/cm 3)
Small, dense, 100 K cool clouds (100 atoms/cm 3)
Warm atomic H clouds slowly cool & contract into denser clouds over m of years
Gravity slowly draws gas together into tighter clumps Energy radiated more efficiently as cloud grows denser
Molecular H 2 forms as atomic H cools from 100 K to 10…30 K.
Molecular clouds created: 70% H2 , 28% He, ~1% CO + many other substances
No emission from H2 !
Cold, heavy & dense molecular clouds settle in the central layers of the Milky Way’s disk
Radio emission of other molecules can be observed.
H2O, CO (3 mm emission line), NH3 , -OH, alcohol
Supernova explosions disturb the cold molecular clouds
→
stir them up & create turbulences
Gravitational push triggers
cloud core formation & their collapse
forms new stars, thereby completing the star-gas-star cycle.
Once a few stars form in a newborn cluster, they begin to hamper the creation of new stars
The molecular cloud is eroded & pushed away by stellar winds & radiation pressure It is also heated/excited & ionized by UV photons from high-mass stars
Supernovae are crucial for both star & planet formation.
Tremendous amounts of matter blasted into intergalactic space
Heavy elements created → the production and dispersal of even a small amount could have had a major effect on star & planets formation
Enable creation of stars from protostellar disks.
When the abundance of ‘metals’ in star-forming clouds rises above one thousandth of the metal abundance in the Sun, the metals rapidly cool the gas to the temperature of the cosmic background radiation
‘Metals’ are much more effective than hydrogen in cooling starforming clouds → faster & easier collapse into stars
Next generation stars begin life with more heavy
elements
→ numerous advantages:
Nuclear fusion in the star core is less efficient without ‘metals’ otherwise it would have to be hotter and more compact to produce enough energy to counteract gravity
Because of the more compact structure, the surface layers of the star would also be much hotter: Tsurf ~100,000 K (17× higher than the Sun’s surface temperature)
The heavy elements also enable the creation of planets & appearance of life
Star-gas-star cycle cannot go on forever.
Stellar formation & evolution -> matter gradually “locked” in red dwarfs, brown dwarfs & stellar corpses
Other material → irretrievably spread out as ISM or intergalactic gas
Star formation rate will taper off over next 50 b years Eventually, star formation will cease!
Galactic recycling in
the Milky Way
The star-gas-star cycle is observed in the Milky Way using many different wavelengths of light
Galactic recycling: SUMMARY
Stars make new heavy elements by fusion.
Dying stars expel gas and new elements , producing hot bubbles of gas (~1m K) which emit X-rays .
This hot gas cools, allowing atomic H clouds to form (~100,000-10,000 K).
Has a 21 cm wavelength emission line. Further cooling molecules (CO, etc.) form, making molecular clouds (~30 K).
Observed using CO emission line at 3 mm.
Gravity forms new stars (and planets) in molecular clouds.
The process starts over again!
Where will our Galaxy’s gas be in 1 trillion years from
now?
A. Scattered in ISM and intergalactic space
B. Locked into white dwarfs and low mass stars
C. Both of the above
D. Blown out of galaxy
E. Still recycling just like now
Galactic recycling is an imperfect process. More and more gas gets locked up into low-mass stars and white dwarfs, which never return their material to the ISM, and some material will remain scattered in space and will never have the possibility to collapse & form stars again
Where do stars form in our galaxy?
In any galaxy, stars -and also regions of new star formation- are not spread evenly (more dense in gas&dust clouds-rich areas) Much of the star formation in the galactic disk happens in the spiral arms.
Spiral arms are enormous & prolific & star formation waves propagating through the gaseous disk
Stars & clouds more densely packed → Gas clouds get squeezed as they move into spiral arms
Squeezing of clouds triggers star formation.
Young stars flow out of spiral arms.
Supernovae of massive stars compress further the clouds, triggering more star formation.
The disk does not appear solid.
Has spiral arms The arms are not fixed strings of stars
Spiral arms are high density waves propagating through the gaseous disk
Stars & clouds more densely packed
Prolific star formation activity
Massive stars, formed as gas clouds pass through spiral arms, die out quickly before completing one galactic orbit → spiral arms appear bluer than the bulge or gaps between arms
Long-lived yellow & red stars survive many galactic orbits & pass through many spiral arms → more evenly distributed throughout the galactic disk
Halo:
Star formation started first in halo, then stopped
NO ionization nebulae, NO blue stars
NO star formation (and hence NO recycling ) →
Only old stars, and with fewer heavy elements (0.02…0.2%)
Galactic disk
Ionization nebulae, blue stars
Star formation active →
Stars of many different ages with ~2% heavy elements
Disk stars formed later & keep forming!
Milky Way’s star formation rate is about 1 MSun/yr.
Conclusion: The halo lacked the gas for new star formation for a very long time because it has settled into the disk
The initial most basic model for Milky Way’s formation
(Monolithic Collapse Model):
It was first thought that the galaxy began as a giant protogalactic cloud containing all the H & He gas that eventually turned into stars or is now present in the ISM.
The protogalactic cloud collapsed and halo stars began to form
Conservation of angular momentum ensured that the remaining gas flattened into a spinning disk. Spiral arms then formed and the star-gas-star cycle started supporting ongoing star formation in the disk.
Multiple merger model: Recent data show that our galaxy formed from a few different gas clouds, not just one.
The earliest stars formed in relatively small clouds, each with a few globular clusters.
These clouds later collided ( rapid star formation in what is now the halo) & created a combined full protogalactic cloud that became the Milky Way.
Once the protogalactic cloud was in place, it collapsed into the disk and the formation of stars & heavy element enrichment proceeded in a more orderly fashion
The evidence for the multiple merger model
the heavy element content of stars in the intermediate layer between the disk & halo depends on their distance from the galactic center
These stars are almost as old as halo stars but formed just before the spinning protogalactic cloud finished flattening into a disk → it had only 10% of the present heavy element content
Old stars in the bulge have a
composition similar to that of the Sun, even though their ages exceed 10 b years
The centre of Milky Way lies in the direction of Sagittarius
.
Bulge obscured by ISM IR & radio views reveals swirling gas clouds & a cluster of several million stars
Bright radio source (with vast threads of emission tracing mgn. field lines) in the centre with occasional Xray flares Sgr A*.
Hundreds of stars crowded within 1 light-year of the region
Mass in Sgr A*
Determined from orbits of fast-moving stars near galactic centre.
Kepler’s Law gives a mass of ~3.7 m Msun
This mass is packed into a space a little larger than our solar system
What is so small, yet so massive?
A supermassive black hole
The black hole at the centre of our
galaxy has an unusually faint X-ray emission
most probably lacks an accretion disk (already gobbled up all material around it)
Life cycles of galaxies are studied the same way as
those of stars:
Impossible to observe continuously from start to end
Observe galaxies at various stages of their lives
Life cycles of galaxies are more difficult to study
Studying the lives & evolution of galaxies is impossible without considering the evolution of the Universe
The study of galaxies is intimately connected with cosmology = the study of the overall structure & evolution of the Universe
Hubble Space Telescope’s long-distance (far field & ultra far field) observations even in regions apparently
utterly empty, devoid of any planets or stars revealed amazing & humbling images of thousands of galaxies.
The images: the farthest we’ve ever seen into the Universe
Over 100 b galaxies estimated in the entire Universe! What did the Deep Field observations tell us?
There is a wealth of distant galaxies to be studied!
