Galaxies and Large Scale Structures Flashcards
(154 cards)
What is the most suitable distance scale and why?
- most conveniently given scale is the speed of light
- needed because of the large size of the galaxy
How fast does light travel?
3 x 10(8) metres per second
Why is it light seconds / minutes / years?
- because the distance is expressed by giving the time required for the light to travel that distance
What is the parallax method?
- as the Earth orbits the Sun, the apparent position of nearby stars seen against the background of very distant stars changes
- the angular displacement of the star being investigated against this background can be measured
- if this angle is not too small to measure accurately, it can give the distance to the star
- if p is half of the angle measured, the distance to the star is proportional to 1/p
- the distance can be expressed as 1/p where p is in seconds of arc
- the resulting unit of distance is called the parsec
- equal to approximately 3.3 light years
What are the basic features of the Milky Way galaxy?
- it is basically disc shaped with a central bulge
- the disc is about 25 kpc to 30 kpc
- this is where new stars are being created
- the stars and dust in the disc are referred to as the disc component of the galaxy
- the disc has several arms and contains open star clusters, loose grouping of stars
- there is also a significant amount of dust and gas in the disc
- there is a central nuclear bulge with a diameter of about 6 kpc
- the disc and nuclear bulge are surrounded by the halo
- this has a spherical distribution of single stars and globular star clusters - called the spherical component of the galaxy
Gas and Dust in the Galaxy
- there are lots of this in the galaxy
- dark patches = gas and dust
- because of this, much of the galaxy, particularly in the plane of its disc, is hidden from observations using normal optical telescopes and visible wavelengths of light
- we get a very incomplete picture of our galaxy if only optical telescopes are used
What features can be seen by optical telescopes?
- MW - the plane of the galaxy but much is hidden & the arm structure is not obvious
- Globular Clusters - mainly away from the plane of the galaxy
- Open Clusters - nearby in the plane of the galaxy in the halo
- Star Associations - nearby in the plane of the galaxy
- Nebulae - emission, reflection and dark - nearby in the plane of the galaxy
What are Open Clusters?
- found in the disc component of the galaxy
- contain about 10 to 10,000 stars
- very variable in size, typically about 25pc across
- have an open appearance since the stars are not crowded together
- loosely gravitationally bound
What are Star Associations?
- groups of stars that have been created recently
- all have similar ages and velocities (proper motions)
- not gravitationally bound unlike the star clusters described about but they all move in approximately the same direction
- associations are part of the disc component of the galaxy
What are Globular Clusters?
- typically contain 10(5) or 10(6) stars in a region that is only 10 to 40 pc across
- have a crowded appearance because the high density of stars within the cluster
- because of the closeness, they are quite tightly bound by gravity
- have populations of stars that are known to be much older than stellar groups found in the galactic disk
- have orbits with a random orientation that can line well out of the plane of the galaxy and are part of the spherical component of the galaxy
What is the Milky Way?
- OUR GALAXY!
- our Sun and Solar system lie within it
- the naked eye shows it in the night sky as an irregular luminescent bad that crosses the sky
- it is formed by thousands of millions of distant stars
How do we see the Milky Way clearly?
- choose a moonless night far away from any street or other light pollution
- allow time for the eyes to become properly dark adapted
How do we explore our galaxy?
- if we only use visible light by using optical telescopes is very limited because of the gas and dust in the galactic
- fortunately visible light is only a small part of the EM spectrum!
- many other parts of the spectrum at different wavelengths, at different wavelengths are now used
What are the different types of EM spectrum and their different telescopes?
1) Radio waves by radio telescopes, eg Jodrell Bank and Aricebo
2) Infra-red radiation - IR telescopes in space and on Earth, eg UKRIT and Spitzer
3) X-rays - x-ray telescopes in space eg CHANDRA and ROSAT
4) Gamma rays - gamma ray telescopes in space eg Compton, Swift Gamma-Ray Burst Mission
RADIO TELESCOPES
- how they investigate
- important wavelengths
- investigate the radio emission from the galaxy at many wavelengths but several are particularly important
> atomic H, 21cm wavelength radio waves
molecular H which radiates at 115 GHz
molecular CO with a characteristic signal at 2.6mm wavelength
INFRA RED TELESCOPES
- how they investigate
- examples
- has a longer wavelength than visible light
- can penetrate the dust that blocks visible light so that it can be used to see into the centre of our galaxy
- also heated dust radiates strongly at some IR wavelengths and this gives a measure of the density of dust in the plane and centre of the galaxy
Earth-based telescopes eg UKRIT
Satellites eg COBE, Spitzer
X-RAY TELESCOPES
- what they are
- examples
- these are emitted by very hot gases and pass easily through the dust
Satellites eg CHANDRA, ROSAT
GAMMA RAY TELESCOPES
- what they are
- examples
- are emitted by unusual high events
Satellites eg Compton
Using Visible Light to give us information about the nature of the galaxy
- because of the dust and gas in the plane of the galaxy its only possible to see a few kpx across
- BUT the O and B class stars are bright and hot and therefore easily seen
- give indications of spiral structures since they are only found in the spiral arms of nearby galaxies and almost certainly in ours
- O and B stars are very short lived and do not exist long enough to escape from the spiral arms where they are formed
- they are in or close to the regions where star formation is taking place
- these stars are often surrounded by ionised, glowing nebulae
Using other parts of the spectrum to give us information about the nature of the galaxy
- its now possible to explore the galaxy using wavelengths away from the visible the spectrum that can pass through the dust and gas more easily
- visual, radio and IR telescopes have given us information about the spiral arm structure
Describe the internal motion of our galaxy
- the stars in he disc are moving in almost circular orbits about the galactic centre
- have orbits in or close to the plane of the disc
- often their motion has a small oscillation perpendicular to the plane of the disc so that periodically, they pass through the thin layer of dust and gas that lies inside the disc, a region called the inner disc
- the Sun moves in the same way
- our solar system is moving at about 220 km/s round the centre of the galaxy
- it takes about 240 million years for one rotation about the centre
- the Solar system is 4.5 thousand million years old
- it has been round the galaxy 15 to 20 years
Describe the differential motion of stars
- at different distances from the centre, the stars of the disc are not moving at the same angular speed
- there is differential rotation unlike all points on a turntable, which have an identical angular speed, ie increasing orbital speed towards the outer angle
How is it possible to estimate the mass of our galaxy?
- we use the velocity of the Sun and the radius of its orbit
- can calculate the mass of the galaxy inside the radius of the Sun’s orbit
- this is the mass that generates the gravitational pull that keeps the Sun in its orbit
- result = 10(11) solar masses
- since there is mass outside the Sun’s orbit, we must concluded that the total galactic mass is significantly greater than this
What is the rotation curve?
- in the general vicinity of the Sun, stars closer to the galactic centre move faster than the Sun and those further away go more slowly
- the way in which the speeds of the disc stars change with the distance from
- the edge of the disc of the galaxy is effectively at about 15 kpc from the centre since there are few stars beyond this distance
- the rotation curve is flat or even increases further out
- this would not occur if most of the mass of the galaxy was concentrated near it centre and indicates there is unseen mass out on the edges of the galaxy
- this is the effect of dark matter, material that is not the type of matter we see in stars and planets