Each of these regions in the electromagnetic spectrum represent transverse waves, travelling as electrical and magnetic fields which interact perpendicularly to each other, with different ranges of wavelength. The magnetic field oscillates vertically and the electric field horizontally, and each field induces the other.

By the end of the nineteenth century, Maxwell gave a realistic value for c, the speed of light:

The relationship between the speed of all electromagnetic radiation, wavelength (l) and frequency (f) is shown to be c = l f.

Because the Universe is so vast, interstellar distances are so great that light emitted can take upwards of millions of years to reach us. Such large distances are often measured in 'light-years'; one light-year (ly) is the distance travelled by a wave of li

ght in a year. Because of the massive speed of light and distances, the light arriving at us would have left the object many years ago, so that looking at a far away star is much like looking back in time.

We know that stars and galaxies are moving away from us, because the spectra lines from some are shown to have been shifted. This is the Doppler effect, where the spectrum lines are displaced, because their wavelengths have been changed. The change in wavelength is related to the velocity:

The Balmer series refers to the emission spectrum of hydrogen, specifically for high energy level electrons dropping back to the second energy level (n=2). Light emitted falls in the visible region of the electromagnetic spectrum, and the intensity of this light is an indication of a star's surface temperature. The Balmer series is due to atoms being excited by kinetic collisions. The electrons of cool atoms occupy their ground state (n=1), as there are few collisions to excite the electrons. The hotter the atoms, the more energetic the collisions; more electrons are excited to even higher levels (n=3, 4,.etc). These electrons now absorb wavelengths beyond the Balmer series. The most intense Balmer emission spectra are from stars with intermediate surface temperatures at around 10 000K. Most electrons can absorb and re-emit wavelengths of the visible spectrum at this temperature.

It is speculated that Ho lies between 40 and 100 km s-1 Mpc-1

Some common words found in the essay are:
Bang Low, Sun Earth, ESA Hipparcus, Observing Stars, Universe Bang, Mpc-1 Doppler, Earth Earth, Earth Unsurprisingly, Tan Distance, T4 Stefan's, electromagnetic spectrum, surface temperature, electromagnetic radiation, light emitted, black body, regions electromagnetic spectrum, doppler effect, radio waves, binary stars, absolute magnitude, surface temperatures, region electromagnetic spectrum, falls visible region, visible region electromagnetic, inverse square law,
Approximate Word count = 2940
Approximate Pages = 12 (250 words per page double spaced)