Galaxies, Stars, and Constellations

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Galaxies, Stars, and Constellations: 

Galaxies, Stars, and Constellations Presented by Group 5


Galaxies are star systems composed of billions of stars. They are considered the basic units of the universe. Edwin Hubble studied about 600 galaxies using a 100-inch optical telescope at Mt. Wilson Observatory. He classified them into three types – Spiral, Elliptical and Irregular. Galaxies

Spiral Galaxy: 

Spiral Galaxies consists of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. Edwin Hubble made a code to identify the types of spiral galaxies: a. Type Sa – spiral galaxy with a large central disk and with arms tightly wound around it b. Type Sb – spiral galaxy with small central disk and spiral arms loosely wound around it c. Type Sc – spiral galaxy with small central disk and spiral arms very loosely wound around it Spiral Galaxy

Spiral Galaxy Type Sa: 

Spiral Galaxy Type Sa

Spiral Galaxy Type Sb: 

Spiral Galaxy Type Sb

Spiral Galaxy Type Sc: 

Spiral Galaxy Type Sc

Elliptical Galaxy : 

Elliptical Galaxies have flat, circular, or ellipsoidal shapes resembling the central disk of spiral galaxies. They are relatively large and massive, with a small amount of interstellar material and composed of main sequence and red stars. The size of elliptical varies from giant elliptical, intermediate and dwarf. Elliptical Galaxy

Irregular Galaxies: 

These are galaxies that lack definite shape or symmetry. They are chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. There are two major Hubble types of irregular galaxies: a. Irr I – an irregular galaxy that features some structure but not enough to place it cleanly in the Hubble sequence. b. Irr II – an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence. Irregular Galaxies

Irregular Galaxy - Irr I: 

Irregular Galaxy - Irr I

Irregular Galaxy – Irr II: 

Irregular Galaxy – Irr II

Hubble Sequence: 

Hubble Sequence

The Milky Way Galaxy: 

The Milky Way is the galaxy in which Earth is contained. It appears like a band because it is a disk-shaped structure being viewed from inside. The fact that this faint band of light is made up of stars was proven in 1610 when Galileo Galilei used his telescope to resolve it into individual stars. In the 1920s, Edwin Hubble showed that the Milky Way is just one of many galaxies. The Milky Way Galaxy


Stars are huge, massive spheres of incandescent gas much like our sun. They begin as clouds of dust and gas and they shine due to the light they produce. It is a massive, luminous sphere of plasma held together by gravity. The nearest star to Earth is the Sun but not counting the sun, the nearest would be the Proxima Centauri. Stars

Properties of Stars: 

Distance: Stars vary in distance. Friedrick Wilhelm Bessel introduced a method of measuring the distances of stars. This method is called the parallax method. The apparent change in the position of the star, due to the change in the position of the observer is called parallax. The greatest distance of stars can also be expressed in another unit called parsec (parallax per second). Properties of Stars

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Magnitude: This refers to the brightest of stars. There are two ways of describing the brightness or magnitude of stars: a. Apparent Magnitude – the brightness of a star as it appears to an observer on earth. b. Absolute Magnitude – the brightness a star would have if it were viewed at equal distance with other stars. The absolute magnitude can be reliably measured using the instrument called photometer. Size: Stars are classified as supergiant, giant and dwarf. The terms describe the size of stars but they do not actually refer to size but the brightness of stars.

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Color and Temperature: The most important factor that affects the color of the stars is temperature. Red stars are relatively cool, bluish-white stars have relatively high surface temperature. Color Red Orange Yellow White Bluish-White Temperature (degrees celsius ) 1,500-3,500 5,000 6,000 7,500-11,000 Over 25,000

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Composition: Most stars are composed of the same materials; 73% hydrogen, 25% helium and 2% all the other elements (iron, nickel, carbon, calcium, titanium, oxide, etc.) Mass: Stars vary in mass. The star with the largest mass is 10 times the mass of the sun while the star with the smallest mass is one fifth that of the sun.

The Life Cycle of Stars: 

The Life Cycle of Stars

The Mass and Evolution of Stars: 

What a star will be as it ages depends primarily on the amount of matter it contains. Some stars live from millions of years and other for trillion of years. Our sun has a life span of 11 billion years. Stars less massive than our sun may undergo a series of evolution similar to our sun. The only difference is that they spend longer time as they pass through the different stages. The Mass and Evolution of Stars

Other Star Classes: 

Binary Stars – a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes or secondary. Other Star Classes

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Variable Stars – stars whose brightness vary periodically. They rhythmically glow brighter and dimmer. a. Cepheid Variable – pulsating star whose brightness and temperature vary according to regular pattern. b. Long Period Variables – stars whose bright and dim periods last for 150 days or more. c. Nova or Supernova – star which suddenly becomes very bright.


Constellation is an internationally defined area of the celestial sphere. There are 88 standard constellations recognized by the International Astronomical Union (IAU) since 1922. The constellations of the zodiac are the most familiar to us. There are about 12 zodiacal constellations. The invention of these twelve constellations goes back to ages of which no record remains. Constellations