- Created by: katie
- Created on: 02-06-14 12:51
Constellations the basics
On cler nights you can see stars as: individual stars, doubble stars(close pairing of two stars), and some are in small open clusters.
Constellations are areas of the sky which contain the pattern of stars refered to as a constellation.
Asterisms are small groups of stars which form shapes like the plough.
Nebulae are stellar nurseries where stars are made, in the sky they look like fuzzy patches of light.
In constellations greek letters can be used to identify each star, alpha is first, beta is second and so on there are some mistakes for example in Orion beta is brighter than alpha.
Basically every culture ever had some form of constellations and all of them have been different.
The first list of constellations was published in 150AD, many of the observations used are thought to come from the Greeks and thenames are thought to come from the general name speople used at the time. the pulisist was Claudis Ptolemaeus.
After that loads of constellations where added and names were changed as they passed through different cultures.
Now the we have 88 constellations which was formally agreed by the International Astronomical Union in 1922.
The Earth orbits the Sun, as it orbits the sun is in the way of different parts of the sky so we see different stars and therefore different constellations at different times of the year. The only stars which are always visible are the stars 'above or below' the ecliptic as the sun never obstructs the view of them.
Observing the basics
When observing you use a grid system much like longitude and latitude but with:
Right Ascention(RA)-east or west
Declanation(dec)-up or down
Stars, galaxies and other distant stuff have set cordinates, the Sun, Moon and other planets don't as they rotate compared with each other.
For finding the declenation, you imagine that the Earth is the center of a giant sphere (celestial sphere) declenation is measured in degrees either + or - from the celestial equator(0 degrees). Max dec for the Sun is +/-23.5 degrees.
Right ascention is like longitude in tha 0 is in a awkward place, 0 RA is the position of the Sun when it moves from the north hemisphere to the southern hemisphere, the position of the sun on the spring equinox. It is also recorded in hours and minutes with 1hour being worth 15 degrees. Max RA for the Sun is 6h-18h.
The ecliptic is the path of the Sun across the sky, the Zodiacal Band spans 8 degrees above and below the ecliptic, it contains all the zodiac constellations.
Polarislies almost directly above the north pole, +90degree dec, great for navigation as it shows north, and can be used to work out latitude- the angle between the northern horizon and polaris is equal to the latitude of the observer.
As the Earth spins the stars appear to rotate in a clockwise direction around polaris, circumpolar stars are stars which don't dip below the northern horizon during the night.
For example if polaris is 60 degrees above the horizon anything more than 60 degrees away from polaris will set at some point.
A star will be circumpolar if:
If you take a long exposure photograph over a set amount of time you can determine the Earth's rotation period.
The equation is:
Rotation period of Earth = 360(degrees)
Exposure time Mean arc angle
In the southern hemisphere it is harder to find the center to measure the arcs, there is no equivalent to polaris. This also makes it harder to work out where you are and it is harder to work out if a star is circumpolar or not.
Stars move westwards.
Stars culminate due south(highest point int he sky). Stars appear brightest at culmination as the atmosphere absorbs the least.
You can predict the time of culmination by using: star charts, computer software, or by knowing RA and the time.
If you want to know when another star will culminate and it RA you can minus the unkown stars RA giving you the angle which can be made into time (1h=15 degrees).
Because it doesn't take 24h to rotate stars culminate 4mins earlier each day.
When deciding when to do your observations you should consider:
1.Phase of the Moon- new Moon will aloow the most stars to be viewed, full Moon will only show the main stars making it easier to identify stars and constellations.
2.Weather forecast-clear nights with no wind are ideal as maximum stars will be visible and having no wind means that there won't be any air turbulance to disrupt the observations.
3.Meteor shower-high chance is good as you can observe that, low chance is good as it means you can observe other things.
4.Comet, planets, or Messier objects-you want a good chance of seeing these to make observations more interesting.
3 and 4, you can find out the chance of seeing these using computer software or month by month guides.
For naked-eye observations you are likely to need:
- Comfy chair
- Torch with a red filter this means it doesn't affect your night vision
- Clipboard, star chart and pen
- Warm clothes
Naked-eye observing is easier and quiccker than using any equimpent.
For the best possible results the eye must be fully relaxed.
The retina is a layer which is photosensitve it is made of two types of cells: 1.Cones-colour sensitive 2.Rods-not colour sensetive
It takes 20-30 mins for your eyes to become cmopletely sensetive to light.
If an object isn't bright enough it won't stimulate the retina's cones, so to view them you must use averted vision-you look slightly to the side of the object which stimulates the rods so you can view the image.
In the sky all the fuzzy objects in the sky are in the Messier Catalougue, these are not really visible with the naked-eye but with binoculars or a telesope they loom amazing.
The catalogue was made by Charles Messier and contains over 100 clusters, nebulae and galaxies.
Star properties the basics
Constellations are not acctually linked in any way, they only appear linked (line-of-sight effect).
However star clusters are linked gravitationaly, they don't just look that way.
Optical doubble stars just appear to really close together.
Binary stars are actually very close and are trapped revolivin each other.
Observed magnitude, if the difference between two stars is 5 the star 5 smaller is 100x brighter.
Origionaly magnitude went from 1-6 with 1 as the brightest, now this has increased to reach far more than 6, it also now streches to minus numbers for some brighter stars and planets. We also now use decimal points to be more specific using computer and more advanced equimpent. A magnitude difference of 1 is 2.5 times brighter/dimmer.
A stars apparent magnitude(m) depends on four things:
1. Energy radiated -size and temp
2. Distance to star
3. interstellar gas and dust
4. Light absorbed by Earth's atmosphere
A stars light inensity or the intensity of any other radiation given of can be worked out using the inverse square law. This means that if a star is 2 times further away its 2x2 dimmer, or if its 5 times futher away its 5x5 dimmer.
True brightness=absloute magnitude(M), this would be the stars apparent magnitude if it was viewed from a distance of 10pc.
Apparent magnitude is liked to absolute magnitudewith the equation below(distance modulus formula):
M=m+5-5 log d log is literally the log key on the calaulator, d= distnace to star(pc)
Stars emit radiation the amount they give of varies regullarly.
The time period could be hours or years.
The two main classes of variable star are:
1. Binary-the two stars orbit each other
2. Cepheid variables-giant yellow stars which contract and expand, the brightness changes with this.
In a binary star system the brighter star is called the primary, the dimmer star is called the companion or secondary. If the stars orbit in line with our plane then when they eclipse each other the bightness increase or decreases
An example of a binary star is Algol.
Capheid variables intensity changes because as it is pulsating the size and temp change these have a massive impact on the luminosity of a star.
It's difficult to measure the distance to stars as you can't travel to them and you can't use radar techniques.
You can't use radar technique sbecause:
1. It would take years for sgnals to come back
2. The signal would be so weak when it returned that you wouldn't be able to islate it from background noise.
3. Stars are made of gas so there is no hard surface o reflect the radar back again
There are two main methods which astronomers do use:
1. Parallax- when you look at the motion of close stars compared th distant fixed stars. You measure where the star is on two occasions when the Earth is in the opposite position. Half the angular shift is the parallax angle and using the formula below canbe used to work our the distance to a star.
d(distance in pc)= 1/p(parallax angle)
2. Cephid variables also allow stars distances to be established, by using the pulsation period and its mean absoloute magnitude.
Evolution of stars the basics
Stars are formed when clouds of dust and gas collapse inwards due to gravitational pull, they are mainly found in the spiral arms of galaxies. The clouds can be up to 15kpc across, thye can have enough suff in them to form thousands of stars.
When a cloud begins to colapse it splits into smaller colapsing knots called protostars, in these the temperature rises as gravitational potential energy is connected to kinetic energy.
The centerof the protostar eventually reaches 15 million K so nucleur fussion begins.
Once nucleur fussion begins the outward pressure from radiation stops it collapsing any more, the star becomes 'stable' and stays at this size.
The next step is for it to radiate energy becoming a main sequence star. Stars can spend 100million years to 1 million million years. Our Sun is halfway through its expected time in main sequence of 10 000 million years.
The hydrogen fuel runs out, this means there is no radiation, the lack of outward force means the star will colapse in on itself again. The nucleur fussion which does occur causes the outer layers of the star to expand and cool. The star are then either red giants or supergiants.
After red giants and supergiants
Red giants- with less and less hydrogen avaliable the temp increases up to 100 mil K, hot enought for helium to form carbon. When it runs out of helium the outside is lost forming a planetary nebula. The inner part collapses forming a hot white dwarf, when it cools it becomes a red then brown then black dwarf.
Superiants-stars of larger masses make supergiants, the temperature at the core is hotter so it can have nucleur fussion up to the element of iron. Once it's used it all it goes supernova in the outer core, and the supergiant blows away the outer layer very quick. The bit left is a supervoa remenant which can either be a neutron star or a black hole.
Neutron stars- mass the same as the sun, ony 20km across, they are under very high gravitational force meaning that thye spin very fast. As they rotate they emit intense radio waves forom there poles this means there are regular pulses of radio waves so astronomers detect them as pulsars.
Black holes-made when there is a greater mass about 3 Sun's, they have very high density and gravitational pull-this means that no light or electromagnetic radiation can escape. You cannot directly observe a black hole, but you can detect x-ray emitted from accreting matter from nearby stars.
You can split light using diffraction grating. This seperates light into a set of abroption lines, every chemical has a different set of spectral lines so you can work out the chemical composition of stars from the absorption lines it presents.
You can also tell the temperature of a star from its spectral lines, it can also be used to work out the radial velocity.
Astronomers use spectral lines to classify stars by their spectral type. The most common used system for classification is the Harvard scheme which uses the letters O,B,A,F,G,K,M (Oh Be A Fine Guy, Kiss Me). O classification is also the hottest going through to M which is the coldest. There is also a number after each letter.
A easy way to study star and there evolution is by using a Hertzsprug-Russell diagram,there are many forms of the chart but the most common form uses two sets of data: luminosity and spectral type/surface temperature.
The chart shows stars fall into four distinct groups:
1. Main sequence
2. Red giants
4. White dwarfs
By using the HR chart you can track a stars progression and predict its evolution in the future.