Topic 3 Mapping The Universe

Question 1

How to measure brightness from image?

Answer 1

You draw a photo metric aperture around the object of interest within which the brightness of each pixel is added together resulting in the total brightness of the object.

Question 2

What is a photometric aperture?

Answer 2

This is a region (often circular) superimposed onto an astronomical image within which the brightness of each pixel is added to give the total brightness.

Question 3

What is a radial surface brightness profile?

Answer 3

This is a measure to decrease with distance from the centre. This can be plotted on a radial surface brightness profile to help determine the size of photometric aperture used.

Question 4

What is point spread function (PSF)?

Answer 4

A mathematical description of the blurring effect caused by the telescope optics and atmospheric effects, so that light from a point like source is spread over multiple pixels of an image.

Question 5

What is a telescopes pixel scale?

Answer 5

This is the conversion factor between the pixels and the angular size or separation in areseconds or arcminuets

Question 6

Calculating inclination angle from observed semimajor axis

Answer 6

θ = cos^-1(b/a)

Where θ is inclination angle, b is the measured semiminor axis and a is measured semimajor axis.

Question 7

Calculating true length from measured length and angle of inclination

Answer 7

L = l / (cosθ)

Where L is the true length, l is the measured length and θ is the angle of inclination.

Question 8

What is an open cluster?

Answer 8

A collection of stars where it is possible to make out individual stars.

Low resolution images may cause stars to blend together forming a nebula like appearance.

Question 9

What is a globular cluster?

Answer 9

This is a collection of stars that are packed more closely together in the cluster centre and are more spread out at larger distances.
The cluster has a round shape and more regular appearance, being brighter in the middle and fainter at the edges.

Question 10

What is a galaxy?

Answer 10

This is a collection of stars, but we cannot identify individual stars. You can see a spiral structure. The smooth diffused nature of the light tells as that galoxys contain far more stars than clusters.

Question 11

What is a planetary nebula?

Answer 11

These nebula have very diverse structures and have nothing to do with planets.

They are the debris of a sun like star that has reached the end of its life.

Question 12

What is a diffuse nebula?

Answer 12

A diffuse nebula encompasses objects where images show a smooth light distribution not made up of individual stars, which is less regular in structure than planetary nebula.

The light from diffuse nebula comes from warm glowing gas associated with star forming regions.

Question 13

What is a narrow band image?

Answer 13

This means that a filter was used that record the light that spans a very narrow range of wavelengths.

Question 14

Why is narrow band imaging useful?

Answer 14

Narrow band imaging allows you to identify which areas of a structure are emitting certain wavelengths of light, allowing you to figure out which areas are more densely populated by particular elements.

Question 15

What are time-domain surveys?

Answer 15

These are images of the same region taken periodically to allow you to see if there is any change in structure or new star or supernova.

Question 16

What are time-domain surveys?

Answer 16

These are images of the same region taken periodically to allow you to see if there is any change in structure or new star or supernova.

Question 17

What is a deep field image?

Answer 17

This is a image of a very small area taken with extremely powerful telescopes with very long exposures that allow you to detect object far further away.

Question 18

What is a brown star?

Answer 18

This is a failed star that have not become hat enough to burn hydrogen.

Question 19

What is a neutron star?

Answer 19

These are very compact dense stars formed in a supernova explosion.

Question 20

What is a globular cluster?

Answer 20

A globular cluster contains tens to hundreds of thousands of stars densely clustered and forming a spherical shape.

Question 21

What is an open cluster?

Answer 21

Open clusters are less lightly bound and typically contain a smaller number of stars (typically around a few thousand).

Question 22

What is a supernova remnant?

Answer 22

The gaseous remains of a stellar explosion, which may be caused by the collapse of a massive star or thermonuclear explosion of a white dwarth.

The shockwave travel through the interstellar medium and can he identified by multiple parts of the electromagnetic spectrum.

Question 23

What is the interstellar medium?

Answer 23

The thinly distributed matter that fills the interstellar space in the Milky Way and other galaxy’s.

It mainly consists of gas and small dust particles (molecular clouds, neutral atomic gas (HI), ionised gas (HII) and dust).

Question 24

What is molecular gas?

Answer 24

Molecular clouds are the densest regions of the ISM, containing very cold gas (T < 50K). At these temperatures hydrogen exists as H2 and other molecules such as CO are present.

H 2 is hard to detect, however CO is easily seen due to strong radio emission lines. This makes CO a good tracer for the locations of cold, molecular material.

Question 25

What is **neutral atomic gas**?

Answer 25

At intermediate temperatures (> 50K) the mainly hydrogen gas exists as atoms instead of molecules, forming neutral atomic interstellar medium.

Question 26

How is neutral atomic hydrogen measured?

Answer 26

Neutral atomic hydrogen (HI) produces a prominent radio emission with a wavelength of 21 cm (21cm line).

Question 27

What is **ionised gas**?

Answer 27

At high temperatures the gas becomes ionised. Ionised hydrogen (HII) is measured from the H alpha line which is produced when a hydrogen atom becomes ionised. Areas of ionised gas are associated with star formation.

Question 28

Why is **dust** important?

Answer 28

Dust is a tiny proportion of the mass of the Milky Way.it consists of tiny lumps of Carbon, silicon, oxygen and other metals. Dust can absorb / block and redden the light from stars (as the dust is heated it re-emitts infrared). The surface of dust is important sites for molecular chemistry necessary for star formation.

Question 29

What are the constituents of a **spiral galaxy**?

Answer 29

- Disc shape (open clusters found) - spiral arms (more active regions of star formation, younger, bluer) - central build (rounded central region, older, redder stars) - stellar halo (extended spheroidal region with lower density of stars & globular clusters) - supermassive black hole - dark matter halo (the farthest region containing matter who's gravitational influence affects stars and gas)

Question 30

Hubble S(x) classification details.

Answer 30

- S0 -thus is the intermediate between an E class and Spiral galaxy. - Sa - distinctly tightly wound arms and a large bright central bulge. - Sb - somewhere in between a Sa and Sc classification - Sc - smaller buldges and less distinctly or tightly wound bulges.

Question 31

Hubble SB(x) classification

Answer 31

SB(x) has the same 4 categories as the S(x) classification (SB0, SBa, SBb, SBc), however they contain a bar like structure through the centre of the galaxy.

Question 32

Why is it thought spiral arms form?

Answer 32

The spiral arms are the region where new stars are forming in gas clouds. As the material at the entry is completing orbits faster than the material at the edge it stretches the gas clouds into spirals.

Question 33

How are elliptical galaxies defined?

Answer 33

Elliptical galaxies are clouds of stars which appear smooth in images without any structural features like spiral arms. They are typically older and redder then spiral galaxy's.

Question 34

Hubble E(x) classification (ellipicity equation)

Answer 34

Elliptical galaxies are classified from E(0) to E(7), where 0 is circular and 7 is very elliptical. They are sorted into these categories mathematically based on now flat they are (f). f = 1 - (b / a) Where a and b are the semi major and semi-minor axis of the ellipse. The category is determined by multiplying f by 10 (i.e. f = 0.6, 0.6 x 10 = E6) Elliptical galaxies are never more elongated than E7.

Question 35

Virial theorem equation (total mass of an elliptical galaxy)

Answer 35

Ek = -(1/2)Eg Where Ek is total kinetic energy and Eg is total gravitational potential energy.

Question 36

Calculate gravitational potential energy (Eg)

Answer 36

Eg = -((GM^2)/R) Where M is total mass and R is radius.

Question 37

Kinetic energy (Ek) equation

Answer 37

Ek = (1/2)mv^2 Where m is mass and v is velocity.

Question 38

Estimating total moss of an elliptical galaxy from velocity dispersion

Answer 38

Δv^2 ~ (GM)/R Where Δv is change in velocity, M is total mass, R is radius.

Question 39

What are **irregular galaxies**?

Answer 39

These are galaxies that have a less regular shape and may appear disrupted or less round in shape without evidence of a bulge or spiral structure.

Question 40

What are **lenticular galaxies** and their Hubble classification (S(0) and SB(0))?

Answer 40

These are classified as S(0) or SB(0). These galaxies are a light disk with no spiral arms with a bulge in the middle. These can be easily missidentified as E class so be careful!

Question 41

What is **lookback time**?

Answer 41

The time it takes for light to reach us from distant astronomical objects.

Question 42

What is the **dropout technique**?

Answer 42

A method for detecting very distant galaxies by comparing their preserve and absence in images taken using different telescopic filters. The cumulative absorbation effects of gos between us and these galaxies mean that galaxies at particular distances are visible in certain filters and missing in others.

Question 43

What is **gravitational lensing**?

Answer 43

The effect where light rays (or any electromagnetic radiation) from a distant source are bent by the gravitational effect of an intervening mass. With strong gravitational lensing, multiple images of a source would be produced. The images are heavily distorted, and typically form ares around the lensing mass. With weak gravitational Iensing there will be mild distortion.

Question 44

What do deep field images tell us about distant galaxies?

Answer 44

Galaxies in the distant universe are generally much bluer. This tells us they are more actively forming stars than nearby galaxies. In the distant universe there are fewer spiral galaxies and many more messy, irregular galaxies.

Question 45

What is a **super massive black hole**?

Answer 45

A massive compact region in the centre of a galaxy only detectable by its gravitational influence.

Question 46

What is a **star forming region**?

Answer 46

A region of goss within the interstellar medium of a galaxy in which stars are in the process of forming.

Question 47

What are **merging galaxies**?

Answer 47

When two or more galaxies are colliding due to their mutual gravitational attraction. They will eventually form a single galaxy.

Question 48

What is **millimetre astronomy**?

Answer 48

Astronomy carried out using the millimetre wave region of the electromagnetic spectrum, which falls between the far infrared and radio regions, and traces emission produced in dusty regions.

Question 49

What are **sub millimetre galaxies**?

Answer 49

A type of distant dusty galaxy that glows brightly at long wavelengths and is forming stars at a very high rate.

Question 50

What is **intracluster medium**?

Answer 50

The gas that his between galaxies within a cluster of glances. Typically such gas is very hot and ionised, and has a very low density.

Question 51

What do radio telescopes show us?

Answer 51

- Show us non-thermal radiation i.e. Synchrotron radiation is emitted as radio produced in magnetic fields. - pulsars ( rapidly rotating neutron stars) emits pulses of radio waves. - 21 cm line shows hydrogen gas. - super nova remnants emit radio waves.

Question 52

What do X-ray telescopes show us? (x - ray binary star systems)

Answer 52

X-rays are emitted by objects that are hundreds of millions of degrees such as stars and extremely hot gas clouds. They can also show **X-ray binary star systems** with a neutron star or black hole where the gas is pulled from the companion star and super heated as it falls into the neutron star / black hole.

Question 53

What do infrared telescopes show us?

Answer 53

The infrared telescopes can see dust clouds and allow you to see inside dust clouds. You con see objects that are thousands of degrees.

Question 54

What is the **sensitivity limit** of a telescope?

Answer 54

The faintest flute that can he detected above background image noise by a particular telescope.

Question 55

What is **spatial resolution**?

Answer 55

A measure of the ability of a telescope to distinguish between closely spaced features in an image.

Question 56

How can the limit of a telescope due to diffraction be calculated? (Rayleigh criterion)

Answer 56

θ = (1.22λ) / D Where θ is the smallest angular separation in radians, λ is the wavelength of observation and D is the size of the telescope dish.

Question 57

What are **corrective optics**?

Answer 57

This is when a telescope can use a reference star to calculate atmosphere disturband disturbance to allow them to distort the mirror accordingly to create a sharp image.

Question 58

How is the presence of **dark matter** implied?

Answer 58

The rotation of galaxies at different radius can be measured and plotted on a radius velocity graph. The graph shows that velocity increases with radius, which is inconsistent with the observed distribution of stars and gas / dust in the galaxy. This implies the presence of dark matter.

Question 59

Other methods of detecting dark matter?

Answer 59

Weighing galaxy clusters via X-ray observations of their gasses. The effects of gravitational Iensing on background galaxies.

Question 60

What are the possible explanations of dark matter?

Answer 60

- Undetected ordinary matter - massive compact objects such as brown dwarths or black holes - neutrinos - weakly interacting massive particles - axions - modified laws of gravity