Topic 3 Mapping The Universe Flashcards
How to measure brightness from image?
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.
What is a photometric aperture?
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.
What is a radial surface brightness profile?
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.
What is point spread function (PSF)?
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.
What is a telescopes pixel scale?
This is the conversion factor between the pixels and the angular size or separation in areseconds or arcminuets
Calculating inclination angle from observed semimajor axis
θ = cos^-1(b/a)
Where θ is inclination angle, b is the measured semiminor axis and a is measured semimajor axis.
Calculating true length from measured length and angle of inclination
L = l / (cosθ)
Where L is the true length, l is the measured length and θ is the angle of inclination.
What is an open cluster?
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.
What is a globular cluster?
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.
What is a galaxy?
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.
What is a planetary nebula?
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.
What is a diffuse nebula?
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.
What is a narrow band image?
This means that a filter was used that record the light that spans a very narrow range of wavelengths.
Why is narrow band imaging useful?
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.
What are time-domain surveys?
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.
What are time-domain surveys?
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.
What is a deep field image?
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.
What is a brown star?
This is a failed star that have not become hat enough to burn hydrogen.
What is a neutron star?
These are very compact dense stars formed in a supernova explosion.
What is a globular cluster?
A ** globular cluster** contains tens to hundreds of thousands of stars densely clustered and forming a spherical shape.
What is an open cluster?
Open clusters are less lightly bound and typically contain a smaller number of stars (typically around a few thousand).
What is a supernova remnant?
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.
What is the interstellar medium?
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).
What is molecular gas?
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.
What is neutral atomic gas?
At intermediate temperatures (> 50K) the mainly hydrogen gas exists as atoms instead of molecules, forming neutral atomic interstellar medium.
How is neutral atomic hydrogen measured?
Neutral atomic hydrogen (HI) produces a prominent radio emission with a wavelength of 21 cm (21cm line).
What is ionised gas?
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.
Why is dust important?
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.
What are the constituents of a spiral galaxy?
- 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)
Hubble S(x) classification details.
- 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.
Hubble SB(x) classification
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.
Why is it thought spiral arms form?
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.
How are elliptical galaxies defined?
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.
Hubble E(x) classification (ellipicity equation)
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.
Virial theorem equation (total mass of an elliptical galaxy)
Ek = -(1/2)Eg
Where Ek is total kinetic energy and Eg is total gravitational potential energy.
Calculate gravitational potential energy (Eg)
Eg = -((GM^2)/R)
Where M is total mass and R is radius.
Kinetic energy (Ek) equation
Ek = (1/2)mv^2
Where m is mass and v is velocity.
Estimating total moss of an elliptical galaxy from velocity dispersion
Δv^2 ~ (GM)/R
Where Δv is change in velocity, M is total mass, R is radius.
What are ** irregular galaxies **?
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.
What are lenticular galaxies and their Hubble classification (S(0) and SB(0))?
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!
What is lookback time?
The time it takes for light to reach us from distant astronomical objects.
What is the dropout technique?
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.
What is gravitational lensing?
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.
What do deep field images tell us about distant galaxies?
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.
What is a super massive black hole?
A massive compact region in the centre of a galaxy only detectable by its gravitational influence.
What is a star forming region?
A region of goss within the interstellar medium of a galaxy in which stars are in the process of forming.
What and merging galaxies?
When two or more galaxies are colliding due to their mutual gravitational attraction. They will eventually form single galaxy.
What is millimetre astronomy?
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.
What are sub millimetre galaxies?
A type of distant dusty galaxy that glows brightly at long wavelengths and is forming stars at a very high rate.
What is intracluster medium?
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.
What do radio telescopes show us?
- 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.
What do X-ray telescopes show us? (x - ray binary star systems)
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 hall where the gas is pulled from the companion star and super heated as it falls into the neutron star / black hole.
What do infrared telescopes show us?
The infrared telescopes can see dust clouds and allow you to see inside dust clouds.
You con see objects that are thousands of degrees.
What is the sensitivity limit of a telescope?
The faintest flute that can he detected above background image noise by a particular telescope.
What is spatial resolution?
A measure of the ability of a telescope to distinguish between closely spaced features in an image.
How can the limit of a telescope due to diffraction be calculated? (Rayleigh criterion)
θ = (1.22λ) / D
Where θ is the smallest angular separation in rations, λ is the wavelength of observation and D is the size of the telescope dish.
What are corrective optics?
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.
How is the presence of dark matter implied?
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.
Other methods of detecting dark matter?
Weighing galaxy clusters via X-ray observations of their gasses.
The effects of gravitational Iensing on background galaxies.
What are the possible explanations of dark matter?
- Undetected ordinary matter
- massive compact objects such as brown dwarths or black holes
- neutrinos
- weakly interacting massive particles
- axions
- modified laws of gravity
