YOUNG STARS:

Here, meaning stars of less than 1 Gy age. A young system is rich in dust and the planets - if any - are still forming (at least the first 100 million years). For M-class stars, actual stellar contraction takes long time and thus they are slightly brighter during this stage. Typical young stars near Earth are Vega and Epsilon Eridani. On the chart, stars with 1 Gy age (and no lower listed) may well still be in the later stages of system formation.

VERY OLD STARS:

Stars that are more than 9-10 Gy old are often very poor in elements heavier than helium. Sometimes these stars are called halo stars or subdwarves, as they belong to the galactic halo and didn't fit exactly into old models of stellar evolution. A very old star close to Earth is Kapteyn's Star, a red dwarf belonging to the halo. However, an old star need not always be poor in heavy elements - supernovae enriched the early galaxy too.

STARS ON THE RIGHT END OF THE CHART:

Rolling a "10" for a star of G0-4 and upward indicates that the star is about to leave the main sequence (class IV-V) and become a subgiant.

GIANTS AND SUBGIANTS:

A typical star spends perhaps 10% of the time as a main sequence star as a subgiant, and slightly less as a real giant. We understand that only the more massive stars (G and up) have had time to enter this stage, and that the less massive a giant or subgiant is the older it is, in absolute age. A subgiant close to Earth is Beta Hydri, which has a spectrum similar to the Sun but is brighter, heavier and at least two billion years older.

WHITE DWARVES:

It might be considered a bit strange that white dwarves with masses below 0.7 exists - after all no giants of so low mass are found and no 0.7 mass star could possibly have evolved fast enough. The explanation is that stars lose a lot of mass due to the often rather pyrotechnic displays during the later stages as a giant, and thus these white dwarves had a much larger mass while they were main sequence stars.

PROTOSTARS:

Are very young. A protostar of similar mass as the Sun is a protostar for about 10 million years and during the later part of that time it is variable and stop diminishing in luminosity. Such stars are called T Tauri stars.

BURSTS OF STAR CREATION:

Some astronomers believe stars are created in much greater numbers during certain periods of the galaxy's history, while in other times star creation is very low. This has not been considered in the chart, however.

LUMINOSITY INCREASE WITH AGE:

A normal star like our Sun continuously becomes brighter. When the system was young, it was perhaps 40% less luminous, and when the Sun leaves the main sequence it will be perhaps 40% brighter. As a subgiant and later as a giant luminosity increase further, and in the giant stage the star grows enough to "eat" any close planets and fry others. The effects of the normal main-sequence increase will be discussed in section ONE/6.

BUT MAY IN SOME CASES DECREASE:

We have already mentioned that white dwarves and brown dwarves cool off with age. This also applies to the smallest red dwarves (less than 0.15 solar masses) which gradually will fade; though it will take many billions of years.

SYSTEM ABUNDANCE:

This tells how rich the system is in heavier elements. It will primarily be used in ONE/4 and ONE/5. Some systems may be rich in specific elements ... a system which has an abundance of carbon (instead of oxygen, like our own) may have different chemical makeup of planets. See the notes on THREE/2 for more information.

PLANETARY NEBULAE:

Really have nothing to do with planets. Instead these are shells of gas ejected by stars during their final stages in life, before becoming a white dwarf or neutron star. More than one such nebulae can be ejected by a large star, but the nebula disperse fairly rapidly (roughly 100 000 years). A typical planetary nebula has a decent mass (0.1 solar masses or more) and a radius of several tenths of LY. Planetary nebulae are uncommon because they survive so short time before dispesing.

POPULATIONS:

Sometimes stars are divided into "young" Population I stars, and "old" Population II stars. Population I is found in the disc of the galaxy, Population II stars in the halo and galactic "bulge".

MOVEMENT:

All stars move as they orbit the center of the galaxy. They also move in respect to each other, so over hundreds of thousands of years the stellar neighbourhood changes. Old stars tend to move more inclined to the galactic plane ... many belong to the galactic halo. Also, the older a star is the more likely it is to have experienced close encounters which have disturbed the orbit. (This in turn makes it likely such stars have lost planets in such interaction). Movement shifts may also be induced by the cataclysmic final stages in a star's life.

STELLAR REGIONS

Spiral Arms:Our sun lies at the edge of a small spiral arm in our galaxy. Spiral arms contain many young stars.

Disc: The entire area of the spiral arms is called the disc. The area of the disc closest to the galactic equator is the place where star formation takes place ... the closer to the upper and lower edges of the disc one get the older the stars are and the more sparse they are.

Bulge: The central region of the galaxy is more spherical and here are many older stars, many rich in heavy elements. Towards the center of the galaxy density of stars increase. This region is comparatively rich in neutron stars, black holes etc.

Halo: The galaxy also is surrounded by a halo of stars ... mostly old stars. The halo is spherical and extend farther than the spiral arms. It has a very low stellar density.

Open Cluster: An open cluster is a group of stars, up to perhaps a thousand, which are young (hundreds of million years, at most). The clusters are found in the spiral arms, and stellar densities are higher than normal (from 2 to 20 times). Old stars may be present in the area too. Open clusters are about 30 LY across.

Globular Cluster: This is a large association of very old stars, found in the halo and bulge. A globular cluster can have 100000 stars within a 100-250 LY sphere. As the stars are so old they are poor in heavy elements and only low-mass main sequence stars remain, the heavy stars have become giants or white dwarves. As the stellar density in a globular cluster can be 1000 times above normal in the spiral arms, any planets originally found are likely to have been disrupted.