Astronomy and related astronomical terms

1. A mass limit for the isothermal, helium core of a main-sequence star above which the star must rapidly increase in radius and evolve away from the main sequence.
2. A limit on the mass of a main sequence star's core above which the star will leave the Main Sequence to become a red giant.

This takes place when the helium core makes up 10 to 15 per cent of he star's mass.

3. The maximum mass of a star's helium-filled core that can support the overlying layers against gravitational collapse, once the core hydrogen is exhausted; it is believed to be 10 to 15% of the total stellar mass.

If this limit is exceeded, as can only happen in massive stars, the core collapses, releasing energy that causes the outer layers of the star to expand to become a red giant.

It is named after Indian-born (Lahore, India, now Pakistan) American astrophysicist, Subrahmanyan Chandrasekhar (1910-1995) and the Brazilian astrophysicist, Mario Schönberg (1914-1990), who were the first to point out this limit and derive it.

1. The twinkling of a point source of light as a result of the turbulence of the atmosphere through which the source's light passes.
2. The twinkling of the stars caused when changes in the density of the earth's atmosphere produce uneven refractions of starlight.
3. A rapid variation in the light of a celestial body caused by turbulence in the earth's atmosphere; a twinkling.

A bright zodiacal constellation in the southern hemisphere between Libra and Sagittarius, represented as a scorpion.

The sun passes briefly through Scorpius in the last week of November. The heart of the scorpion is marked by the bright red super giant star Antares.

Scorpius contains rich Milky Way star fields, plus the strongest X-ray source in the sky, Scorpius X-1. The whole area is rich in clusters and nebulae.

1. Half the length of the major axis of an ellipse; a standard element used to describe an elliptical orbit. 2. Half of the larger diameter of an ellipse.

Half of the smaller diameter is the semi-minor axis.

A navigational instrument for determining latitude by measuring the angle between some heavenly body and the horizon.

It was invented in 1730 by John Hadley (1682-1744) and can be used only in clear weather. John Hadley was an English mathematician, inventor of the octant (reflecting quadrant) and precursor to the sextant around 1730.

1. A type of spiral galaxy first discovered by U.S. astronomer, Karl Seyfert (1911-1960), in the 1940s.

The central region of a Seyfert galaxy is distinguished by powerful radiation, much of it focused into narrow frequencies.

2. A galaxy with an unusually bright central nucleus, often emitting strongly in the infrared region of the electromagnetic spectrum, as a result of hot dust within it.

About two per cent of all galaxies are Seyferts.

3. A spiral galaxy with a small, compact, bright nucleus that exhibits variable light intensity and radio-wave emission.

A popular name for a meteor.

The time required for a celestial body in the solar system to complete one revolution with respect to the fixed stars (as observed from a fixed point outside the system).

A planet's sidereal period can be calculated from its synodic period or the length of time during which a body in the solar system makes one orbit of the sun relative to the earth.

The sidereal period of the moon or an artificial satellite of the earth is the time it takes to return to the same position against the background of stars.

1. Time that is measured by the stars rather than by the sun.
2. A unit of time used in astronomy, equal to the period of time in which the earth makes one rotation relative to the stars.
3. The time taken between successive meridian passages of the same star (23 hours, 56 minutes, 4.091 seconds).

The sidereal day is about four minutes shorter than the twenty-four hours of the solar day.

The class of iron meteorites, which includes those with about 90 percent iron and 10 percent nickel.

The class of story-iron meteorites, that includes those with about 50 percent silicates and 50 percent iron and nickel.

A large group of minerals containing silicon and oxygen, usually combined with one or more metals and most common rocks are silicates.

1. In astrophysics, the point in space-time at which the known laws of physics break down.

Singularity is predicted to exist at the center of a black hole, where infinite gravitational forces compress the infalling mass of a collapsing star to infinite density.

It is also, according to the Big Bang model of the origin of the universe, to be the point from which the expansion of the universe began.

2. A point in space at which an infinitely strong gravitational field exists.

Such a concept is predicted by general relativity to exist at the center of a black hole.

A measure of the year based on earth's revolution around the sun, which takes 365.2422 days.

This is the calendar used in most of the world today.

The interval between two successive noons.

Since the sun is moving across the sky background at a rate of about one degree per day in an easterly direction, the solar day is slightly longer than the sidereal day; also, the sun's rate of motion varies, and it is greatest at perihelion.

The mean solar day is equal to 24 hours, 3 minutes, 36.555 seconds.