The object may be:
Reminder: the hotter the object, the shorter the wavelength of maximum emission.
Lambda (max) = 3,000,000 nanometers / T
(T is the temperature in degrees Kelvin)
A star is a hot, dense object. Therefore, measure Lambda (max) and you can find the temperature of the star.
Two stars in the constellation Orion:
An atom consists of a nucleus (containing protons and neutrons) surrounded by a swarm of electrons
Photons are emitted as electrons go from one permitted energy level to another.
Only photons of certain specific energies can be emitted. For instance, hydrogen atoms emit photons with wavelength of 410 nm, 434 nm, 486 nm, and 656 nm.
The high-energy atoms in a HOT gas EMIT photons of specific energies
The low-energy atoms in a COOL gas ABSORB photons at the same energies.
One element, one spectrum.
The atoms of each element have different energy levels, and therefore emit (or absorb) photons of different energies.
The spectrum of each type of atom is as individual as a fingerprint. It's the bar code, or DNA fingerprint of that particular element.
Removing electrons from an atom (converting it to an ion) changes its absorption and emission spectra.
Bonding an atom into a molecule also changes its absorption and emission spectra.
By observing the spectrum of a low-density gas, you can determine what its chemical composition is.
The outer layer of a star's atmosphere consists of cool, low-density gas.
Therefore, by observing the absorption lines, we can see which atoms, ions, and molecules are present.
In the Sun's atmosphere, for instance, 92.0% of the atoms are Hydrogen; 7.8% are Helium; the remainder are heavier atoms.
A light source is moving toward you: A shift to SHORTER wavelength (yellow becomes blue).
A light source is moving away from you: A shift to LONGER wavelength (yellow becomes red).
The shift in wavelength is proportional to the radial velocity:
V/c = (Delta Lambda)/Lambda
Note that radar traps on the highway work by measuring the Doppler shift of radio waves bounced off your car.
V = 66 miles/hour = 0.03 km/sec
c = 300,000 km/sec
(Delta Lambda) / Lambda = V/c = 1/10,000,000
The radar guns must measure a Doppler shift of 1 part in 10 million!
More seriously, astronomers, by measuring the spectrum of electromagnetic radiation emitted by a star, can determine its
even though the star may be hundreds of light years away.