H-R Diagram in Astronomy

Hertzsprung-Russell Diagram

• The H-R diagram is a graph that shows the effects of temperature and luminosity on a star.
• It was devised by Hertzsprung and Russel in the 1900s to organize stars.
• About 90% of stars fall along the main sequence, which is a line of stars down the center of the H-R diagram.
• Stars outside the main sequence include giants, supergiants, and dwarf stars.
• As a star ages, it loses energy and moves to different locations on the H-R diagram.

Galaxies

• Galaxies are massive clusters of stars, dark matter, nebula, and interstellar clouds of gas and dust bound together by gravity.
• Most large galaxies contain a supermassive black hole.
• There are potentially 100-200 billion galaxies in the observable Universe.
• Hubble invented a classification system for galaxies, dividing them into three main types: spiral, elliptical, and irregular galaxies.
• Spiral galaxies have subtypes, such as barred spiral and lenticular spiral.
• Elliptical galaxies can be subdivided into categories based on their appearance, from E0 to E7.
• Irregular galaxies are smaller and can be subdivided into irregular I and irregular II categories.

Galaxy Structure

• Galaxies have a disk component where star formation occurs and a spherical component.
• The spherical component includes the halo and central bulge.
• Globular clusters of stars occur in the spherical component and contain old stars.
• Open clusters of stars occur in the disk and contain young stars.

Milky Way

• The Milky Way is a barred spiral galaxy.
• It is 100,000 light-years across and contains anywhere from 100 to 400 billion stars.
• At the core of the galaxy is a supermassive black hole.
• The galactic bulge is a dense region of stars, many of which are extremely old.
• The sun is located in one of the arms of the galaxy.

Magnetism

• The motion of electrons moving around the nucleus of an atom creates an electric current.
• When electrons spin in the same direction, they become strongly magnetic.
• A strong magnetic material must come close to the magnetic field of a substance to magnetize or induce magnetic properties.
• The magnetic field is the region around a magnetic material where the force of magnetism acts.
• All magnets have magnetic poles, which are the regions at the end of a magnet where the magnetic field is strongest.

Earth's Magnetic Field

• The Earth's outer core is made of molten iron and nickel.
• The movement of energy in the core generates electric currents, forming a natural magnetic field, called the geomagnetic field, that surrounds the Earth.
• The mechanism of producing the Earth's magnetic field is known as the dynamo or geodynamo.
• Earth's geomagnetic poles are the areas where the magnetic field is strongest.
• The geomagnetic poles are imagined from a hypothetical model and don't always line up with the magnetic poles.

Orbital Motion

• Objects orbit other objects when the force of inertia and gravity are balanced.
• The orbits of the planets around the Sun are not in perfect circles, but rather are elliptical in shape.
• The amount of deviation from a perfectly circular orbit is called orbital eccentricity.
• The Earth spins on its axis and orbits the Sun.
• The Earth's axis is tilted about 23.4 degrees off perpendicular to its orbital plane.

Seasons

• Seasons are divisions of the year that tend to have distinctive types of weather and hours of daylight.
• The reason for seasons is the tilt of the Earth's axis.
• The Northern Hemisphere is tilted towards the Sun in the summer and tilted away in the winter.
• The tilt of the Earth's axis causes the seasons.
• The angle of the Sun's rays on the Earth determines the seasons.

Cycle of the Seasons

• The vernal (or March) equinox is the time when spring begins in the Northern Hemisphere and fall in the Southern Hemisphere.
• The summer (or northern) solstice occurs around June 21, and it marks the beginning of summer in the Northern Hemisphere and winter in the Southern Hemisphere.
• The autumnal (or September) equinox is the time when autumn begins in the Northern Hemisphere and spring in the Southern Hemisphere.
• The winter (or southern) solstice is the time when winter begins in the Northern Hemisphere and summer begins in the Southern Hemisphere.

Natural Satellites

• A natural satellite is a heavenly body that orbits a planet.
• The Moon is a natural satellite that receives its light from the Sun and reflects it onto the Earth.
• The Moon takes approximately 29.5 days to complete one revolution around the Earth.
• The Moon's orbit is inclined by a few degrees to the Earth's orbital plane.

Eclipses

• Eclipses are when an observer can see the light of one celestial object become obscured from another celestial object passing by.
• There are two types of eclipses: solar and lunar eclipses.
• A solar eclipse occurs when the Moon passes in front of the Sun.
• A lunar eclipse occurs when the Earth passes between the Sun and the Moon.
• There are three types of solar eclipses: total, partial, and annular.
• The longest possible time a solar eclipse can last is approximately seven minutes and thirty-one seconds.

Lunar Eclipses

• A lunar eclipse occurs when the Moon passes through the Earth's shadow.
• There are four types of lunar eclipses: total, partial, penumbral, and central.
• The umbra is the darkest shadow produced when the Moon entirely covers the Sun.
• The penumbra is the lighter shadow produced when the Moon partially covers the Sun.
• Totality is the period of time that the Moon spends completely inside the umbra.

Eclipse Prediction

• Ancient people could predict future eclipses by looking for patterns and cycles in their appearances.
• The Saros cycle is an 18-year, 11-and-one-third-day cycle and pattern of solar and lunar eclipses.
• An eclipse can only occur when the Moon passes through the plane of the Earth's orbit.
• The line of nodes is the imaginary line that connects the two nodes of the Moon's orbit around the Earth.
• Eclipse seasons are the times when eclipses of the Moon and Sun are possible.