The milky way
Download
1 / 59

spr089 - PowerPoint PPT Presentation


  • 369 Views
  • Uploaded on

The Milky Way Dr Bryce 29:50 Class notices Homework: We are moving towards the end of semester, it is vital that you maximise your grade by completing all your homework CSP observing exercise Exam behaviour The Milky Way galaxy appears in our sky as a faint band of light “All sky view”

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'spr089' - Audrey


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
The milky way l.jpg

The Milky Way

Dr Bryce

29:50


Class notices l.jpg
Class notices

  • Homework: We are moving towards the end of semester, it is vital that you maximise your grade by completing all your homework

  • CSP observing exercise

  • Exam behaviour



Slide4 l.jpg


Slide5 l.jpg

Dusty gas clouds obscure our view because they absorb visible light

This is the interstellar medium that makes new star systems


Interstellar medium l.jpg
Interstellar Medium visible light

  • Can both absorb and emit light

  • Most of the interstellar medium is gas and it is easiest to observe when it forms an emission cloud/nebula

  • Good examples of this include the Orion Nebula

  • Because the gas is predominantly hydrogen we see lines associated with atomic or ionized hydrogen


Slide7 l.jpg

We see our galaxy edge-on visible light

Primary features: disk, bulge, halo, globular clusters


Globular clusters l.jpg
Globular clusters visible light

  • We know from our H-R diagrams that globular clusters are old

  • One way to map the Milky Way is to consider the distribution of globular clusters




Slide11 l.jpg




Slide15 l.jpg

Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit:

1.0 x 1011MSun

Sun is about 8kpc from the galactic centre


Orbital velocity law l.jpg
Orbital Velocity Law within Sun’s orbit:

  • The orbital speed (v) and distance from the galactic centre (d) of an object on a circular orbit around the galaxy tells us the mass (M) within that orbit


Slide17 l.jpg

Star-gas-star cycle within Sun’s orbit:

Recycles gas from old stars into new star systems



Hii regions l.jpg
HII regions of hot gas

  • “H two”

  • Strong emission lines

  • A central hot star emits UV photons which ionize the hydrogen

  • When an electron is recaptured by a proton the HII line is emitted


Hii regions20 l.jpg
HII regions of hot gas

  • Require a hot star to have formed in a molecular cloud

  • The hotter the star the larger the HII region can be

  • HII regions tend to be red – see the Rosette Nebula




Slide23 l.jpg


Slide24 l.jpg

Supernova remnant cools and begins to emit visible light as it expands

New elements made by supernova mix into interstellar medium


Slide25 l.jpg

Radio emission in supernova remnants is from particles accelerated to near light speed

Cosmic rays probably come from supernovae


Slide26 l.jpg

Multiple supernovae create huge hot bubbles that can blow out of disk

Gas clouds cooling in the halo can rain back down on disk


Slide27 l.jpg

Atomic hydrogen gas out of disk forms as hot gas cools, allowing electrons to join with protons

Molecular clouds form next, after gas cools enough to allow to atoms to combine into molecules


Slide28 l.jpg




Gas recycling l.jpg
Gas recycling star-forming clouds

  • Stars make new elements by fusion

  • Dying stars expel gas and new elements, producing hot bubbles (~106 K)

  • Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K)

  • Further cooling permits molecules to form, making molecular clouds (~30 K)

  • Gravity forms new stars (and planets) in molecular clouds

Gas Cools


Interstellar gas temperature l.jpg
Interstellar gas temperature star-forming clouds

  • Molecular clouds are dense and at low temperatures (~10K)

  • Interstellar gas is much less dense and much warmer (~10,000K)

  • We also see very hot (~1 million K) gas from Supernova shock waves, it is these regions that are responsible for the X-ray bubbles


Slide33 l.jpg


21cm line l.jpg
21cm line star-forming clouds

  • Associated with the lowest energy level of Hydrogen

  • Doesn’t involve the hydrogen atom interacting with another photon so we can “see” this line anywhere in space


Dark nebula l.jpg
Dark Nebula star-forming clouds

  • Associated with interstellar dust

  • Dust particles block the photons from the stars behind them

  • Dust will re-emit in the infra-red


The development of our model l.jpg
The development of our Model star-forming clouds

  • Galileo first observed that the Milky Way is made up of stars and many astronomers have tried to map it

  • For example Herschel used star counts, see below


Early models l.jpg
Early models star-forming clouds

  • Were incorrect as they didn’t include the effects of interstellar dust which will dim starlight (this effect is called extinction) and interstellar reddening

  • It is for these reasons that we actually find it easier to study other galaxies rather than the galaxy in which we live


Slide38 l.jpg

We observe star-gas-star cycle operating in Milky Way’s disk using many different wavelengths of light


Slide39 l.jpg

Halo: No ionization nebulae, no blue stars disk using many different wavelengths of light

 no star formation

Disk: Ionization nebulae, blue stars  star formation


Slide40 l.jpg

Halo Stars: disk using many different wavelengths of light

0.02-0.2% heavy elements (O, Fe, …),

only old stars

Halo stars formed first, then stopped

Disk Stars:

2% heavy elements,

stars of all ages

Disk stars formed later, kept forming


Slide41 l.jpg

Much of star formation in disk happens in spiral arms disk using many different wavelengths of light

Whirlpool Galaxy


Spiral structure l.jpg
Spiral Structure disk using many different wavelengths of light

  • We can easily observe spiral arms in other galaxies but within the Milky Way our view is hindered by the effects of interstellar gas and dust


Slide43 l.jpg

  • Spiral arms are waves of star formation disk using many different wavelengths of light

  • Gas clouds get squeezed as they move into spiral arms

  • Squeezing of clouds triggers star formation

  • Young stars flow out of spiral arms


Density waves l.jpg

Stars slow down in the spiral arms disk using many different wavelengths of light

Density Waves


Slide45 l.jpg

Our galaxy probably formed from a giant gas cloud disk using many different wavelengths of light


Slide46 l.jpg

Halo stars formed first as gravity caused cloud to contract disk using many different wavelengths of light


Slide47 l.jpg

Remaining gas settled into spinning disk disk using many different wavelengths of light


Slide48 l.jpg

Stars continuously form in disk as galaxy grows older disk using many different wavelengths of light


Cloud collisions l.jpg

Collisions cause the flattening of the disk disk using many different wavelengths of light

Upwards or downwards motions tend to be cancelled out

Cloud collisions


Rotation l.jpg
Rotation disk using many different wavelengths of light

  • Possible models for rotation

  • Wheel or Merry-go-round

  • Planetary or Keplerian

  • Milky Way doesn’t rotate like either of these models


Milky way s rotation curve l.jpg
Milky Way’s rotation Curve disk using many different wavelengths of light

  • Is “flat”

  • This means that the distribution of mass in the Milky Way continues outwards past the luminous material (stars)

  • The dark matter could be brown dwarfs, white dwarfs, Jupiters, Black holes or elementary particles, they are not emitting light but they are exerting gravitational influence



Slide53 l.jpg

We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H


Slide54 l.jpg

Spiral galaxies using the Doppler shift of the 21-cm line of atomic Hall tend to have flat rotation curves indicating large amounts of dark matter


Gravitational microlensing l.jpg
Gravitational microlensing using the Doppler shift of the 21-cm line of atomic H

  • A dark object in the galactic halo (MACHO) could act as a lens because of the curvature of spacetime around it.

  • Black holes would be the strongest type of microlens


Slide56 l.jpg

Infrared light from center using the Doppler shift of the 21-cm line of atomic H

Radio emission from center


Slide57 l.jpg

Swirling gas near center using the Doppler shift of the 21-cm line of atomic H

Orbiting star near center


Slide58 l.jpg

Stars appear to be orbiting something massive but invisible … a black hole?

Orbits of stars indicate a mass of about 4 million MSun


Slide59 l.jpg

X-ray flares from galactic center suggest that tidal forces of suspected black hole occasionally tear apart chunks of matter about to fall in


ad