Chapter 6 : The Galaxy and Night Sky
Earth and the other planets that orbit the Sun and together form our solar system.
The Sun is relatively a small star and only one of the billions.
The Milky Way is the galaxy that contains our Solar System.
The galaxy has been found to be made up of countless individual stars that seem relatively close together.
Topics covered in this snack-sized chapter:
The spiral is the main type of galaxy in the universe.
Spiral galaxies have arms that form a spiral pattern around a central bulge.
The arms form a disk around the nucleus as the nucleus spins, the arms follow behind it.
The youngest stars in the spiral galaxies are found in loosely packed arms.
Older stars lie mainly in the dense nucleus.
The oldest star of all resides in a sparsely populated spherical halo that surrounds the galactic disk.
The Andromeda galaxy is a spiral galaxy and is the furthest object visible to the naked eye on Earth.
The arms also contain much gas and dust that have yet to form into stars.
A Barred-spiral galaxy has an elongated, bar-shaped central bulge.
While the nucleus rotates, an arm seems to follow at each end.
Spiral and barred-spiral galaxies ranges in shape from those with large, central bulges circled by tightly bound arms, to those with small bulges and loose arms.
Irregular galaxies have no regular shape or structure.
They are typically less massive than other galaxies, and most of their stars are bright and young.
Although many irregular galaxies contain regions of luminous gas in which stars are being born.
Although irregular galaxies make up about 25 % of all known galaxies.
Only about 5 % of the 1000 brightest galaxies are irregular.
Elliptical galaxies range in shape from ellipsoidal to spherical.
Unlike other galaxies, in which dust reflecting from hot young stars can be seen, elliptical galaxies appear yellow.
This is because the process of star formation in them has stopped, so that nearly all their light comes from old red-giant stars.
Both the least and the most massive galaxies observed so far are elliptical.
The regions between stars in a galaxy are filled by the interstellar medium.
It consists mainly of hydrogen and helium gases, with traces of other gases and a tiny amount of dust.
This material is uneven in distribution and temperature.
Its density is many billions of times less than that of air.
Much of the interstellar medium (ISM) consists of clouds.
- Some of which can be detected as nebulae if they emit or reflect light from stars.
The Celestial Sphere:
- An imaginary sphere surrounding the Earth.
- An axis through Earth’s North and South Pole also passes through the celestial North and South Pole.
- Horizon – the plane you stand on.
- Zenith – the point right above you.
The sky is like a hollow sphere with the stars attached to it.
This sphere rotates once every 24 hours.
- Meridian – the line from North to Zenith and then to South.
It is useful to imagine the Earth as being stationary while the celestial sphere rotates around it.
The north celestial pole is directly above the North Pole on the Earth.
The south celestial pole is directly above the South Pole on the Earth.
The celestial equator is an extension of the Earth’s equator on the sky.
The zenith is the point, directly overhead.
- This imaginary sphere is called the Celestial Sphere.
The celestial poles and the celestial equator are the same for everyone.
The zenith and the horizon depend on the point where we stand.
- The horizon is the circle, 90 degrees from the zenith.
A group of stars forming a pattern that is traditionally named after its apparent form or identified with a mythological figure.
People have long made up stories about groups of stars that appear close together in the sky. Such groupings are called Constellations.
The sky was officially divided up into 88 constellations so that a star is associated with only one constellation.
The modern constellations have strictly defined boundaries by international agreement.
The stars are usually not physically associated with each other.