Constellation - An area of the sky containing a pattern of stars. There are 88.
Asterism – A pattern of stars in the sky, e.g. the Plough
Open cluster – a group of young stars, e.g. the Pleiades
Globular cluster – a group of old stars tightly packed around the galactic nucleus that resemble a fuzzy ball, e.g. M13
Nebulae - a dense cloud of gas and dust that appear like faint fuzzy patches of light
Double Stars – Stars that appear close to each other. Some are binary stars that are close and orbit each other. Whereas others just appear close because of the angle we see them at.
Observing the Night Sky
Right Ascension & Declination
Right Ascension is measured from the vernal equinox (were the sun appears to cross the celestial equator) in a similar way to longitude. Right Ascension is sweeps eastward like bearings on a compass. It is measured in hours, minutes and seconds with 15 degrees equal to one hour.
Declination is measured from the celestial equator in a similar fashion to latitude. It is measure in degrees.
The Right Ascension and Declination of stars, nebulae and galaxies are fixed but the Sun, moon and planets are not due to their orbits.
The declination of Polaris is 90 degrees this means that it appears to stay in a fixed position in the sky. The angle of Polaris is equal to the latitude of the observer.
Circumpolar Star – A star that does not dip below the horizon from the perspective of the observer
You can find out if a star is circumpolar using the following formula:
Declination of Star >= 90 – latitude
This is because the latitude is the distance the horizon is away from Polaris and as long as the stars declination is higher than this then it will be circumpolar.
Any constellation that is not circumpolar is a seasonal constellation. This means that they might only be visible for a few months of the year like Orion.
Created by Charles Messier, a list of 110 indistinct objects in the night sky that are not comets. Made as he was fed up of discovering objects that looked like comets but were not. Contains fine examples of star clusters, nebulae and galaxies but was limited to objects that could be seen from France.
Physical Properties of Stars
- Stars in a constellation are not gravitationally related unlike those in a cluster that are
- Optical double stars merely appear to be closely-aligned whereas stars in a binary system are the same distance from us and orbit their common centre of gravity
- The apparent magnitude of a star is the measure of how bright it appear in the sky
- This is influenced by:
- the total energy radiated by the star (dependant on size and temperature)
- the distance to the star
- the amount of interstellar gas and dust
- the amount of light absorbed by Earth’s atmosphere
- It is measured on a logarithmic scale of 2.5
- Absolute magnitude is the magnitude (brightness) of a celestial object as it would be seen at a standard distance of 10 parsecs
- This is calculated using:
M = m + 5 - 5 log D
M = Absolute Magnitude
m= apparent magnitude
- The intensity of light obeys the inverse square law
- Distances to stars are measured using heliocentric parallax
- This involves measuring the change in position relative to background stars when on opposite sides of the sun
- Half of the angular shift is the parallax angle and the inverse of the parallax angle is the distance to the star in parsecs
- A parsec is the distance at which a star would have a parallax angle of one arcsec
- A Cepheid variable can also be used to calculate distances as the period of the star is related to its absolute magnitude and we can observe the apparent magnitude
- Using the formula above we can then calculate the distance to the star
- A star that pulses frequently resulting in a changes in luminosity over a few days
- The longer the period of the star the brighter the star is
- The period is measured between the peaks of brightness
- The brighter star is called the primary and the dimer the companion or secondary
- If the two stars eclipse each other it will produce a light curve
- A period is competed when the stars are back in the same positions
- A spectrometer can be attached to a telescope
- This produces a spectrum of light from the telescope which has dark absorption lines
- These lines correspond to atoms in the outer regions of the star absorbing light at well-defined wavelengths
- Spectrums can be used to deduce the chemical composition of the star
- They can also be used to find the temperature (and so colour) and radial velocity using the Doppler effect
- Stars are classified according to their spectral type