Constellations A constellation is simply a grouping, or pattern, of stars. In ancient times, constellations only referred to the brightest stars appearing to form groups. They were believed to represent great heroes and mythological figures. Today, they are well defined regions of the sky, regardless of the presence of bright stars.
Constellations There are 88 official constellations we study today. In addition to those, the sky contains a number of less formally defined groupings called asterisms. Example 1: The Big Dipper is a well-known asterism of the constellation, Ursa Major(The Great Bear).
Constellations Example 2: The Great Square of Pegasus (includes stars from both the Pegasus and Andromeda constellations).
Constellations The stars of constellations only appear to be close together. This is often known as the projection effect. They may actually be located at very different distances from us.
Constellations and Their Stars
Naming Stars In addition to naming groups of stars, ancient astronomers gave individual names to the brightest individual stars. They are named by a Greek letter (α, β, γ) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Example: brightest star in constellation Canis Majoris alpha CanisMajoris (αCanisMajoris). Brightest star in a constellation is usuallyalpha (α), the second brightest beta (β), and so on in descending order.
Naming Stars In the constellation, Orion, however, that is not the case. βOrionis is actually brighter than αOrionis. αOrionis =Betelgeuse βOrionis =Rigel Betelgeuse Alnitak Alnilam Mintaka Rigel
Brightness of Stars Astronomers measure the brightness of stars using what is known as the magnitude scale. Ancient astronomers divided this scale into 6 classes. The brightest: 1st magnitude stars, those fainter were known as 2nd magnitude stars, etc. in order of decreasing brightness down to 6th magnitude (faintest visible to the unaided human eye). Sometimes, stars are so bright, they extend into the negative numbers. Example: Sirius (brightest star in the sky) = -1.47 We also extend the faint end of the scale for stars extremely faint to us, requiring us to use a telescope. These numbers are known as apparent visual magnitude, (mv), describing how stars look to human eyes observing from Earth. The actual magnitude of a star from a standard distance is known as the absolute visual magnitude, (Mv).
Brightness of Stars
Magnitude and Intensity Flux is a measure of the light energy from a star hitting one square meter in one second. Defines the intensity of starlight. If 2 stars have intensities IA and IB, the ratio of their intensities is IA/IB. Astronomers have defined the magnitude scale so two stars differing by 5 magnitudes have an intensity ratio of 100. Therefore, stars only differing by 1 magnitude must have an intensity ratio of 5√100, which equals 2.512. The light of one star is 2.512 times more intense. Intensity Ratio Formula IA IB = 2.512(m –m) A B
Magnitude and Intensity Examples: Two stars differ by 6.32 magnitudes. What is their intensity ratio? Two stars differ by 4 magnitudes. What is their intensity ratio?
Magnitude and Intensity What if we know the intensity ratio and want to find the magnitude difference? Difference In Magnitude Formula IA IB
Light from the Sun is 24.2 times more intense than light from the North Star, Polaris. What is the magnitude difference between the two stars.
( ) mB – mA = 2.5 log
The Celestial Sphere
The Celestial Sphere
The Celestial Sphere The distance between two stars on the celestial sphere can only be given as the difference between the directions in which we see the stars. Therefore, they are measured as angles, either in: degrees (o): Full circle = 360o arc minutes (‘): 1o = 60’ arc seconds (“): 1’ = 60”
The Celestial Sphere From geographic latitude l (northern hemisphere), you see the north celestial pole l degrees above the northern horizon. From geographic location –l (southern hemisphere), you see the south celestial pole l degrees above the southern horizon. Celestial equator culminates 90° - l above the horizon. 90o - l l
The Celestial Sphere
Apparent Motion Looking north from mid-northern latitudes, some stars appear to circle around the north celestial pole.
Apparent Motion Constellations that never rise or set are called circumpolar constellations. Below are the 5 we see from our latitude: Ursa Major Ursa Minor Cepheus Cassiopeia Draco
The Sun and Its Motions Earth has 3 distinct motions: Rotation – the turning, or spinning, of a body on its axis. Revolution – the motion of a body, such as a planet or moon, along its orbit around some point in space. Precession – the wobbling of a body around its axis of rotation.
Rotation The main results of Earth’s rotation (W to E) is day and night. Rotating @ 1000 mph. Axis of rotation tilted about 23.5°
Synodic vs. Sidereal Day We can measure Earth’s day in 2 ways: Synodic (Solar) – one complete rotation with respect to the Sun: 24 hours Sidereal – one complete rotation with respect to distant stars: 23 hours, 56 minutes
Revolution Earth’s revolves around the Sun in a very slight elliptical orbit. Revolving at an average speed of 67,000 mph. Average distance of 93 million miles. Due to its slightly elliptical orbit, the distance will vary: Perihelion – Earth is closest to the Sun (91 million miles in January). Aphelion: Earth is farthest from the Sun (95 million miles in June).
Annual Motion of the Sun During the day, the Sun appears to move across the sky once it rises in the East until it eventually sets in the West. Over the course of a year, the Sun appears to move East against the background of the stars. The apparent path of the Sun against the background of the stars is called the ecliptic. If the sky were a great screen, the ecliptic would be the shadow cast by Earth’s orbit. Stars are present during the day (not visible due to Sun). Certain constellations appear in the ecliptic path at different times of the year (sun is blocking them) and are known as the zodiac constellations. Recently, astronomers have pointed out the original zodiac constellations may be outdated since they were created 3000 years ago and the Earth has moved slightly due to precession. Believed that the Sun moves into a new constellation approx. every 2160 yrs.
Seasons on Earth Earth’s seasons are caused by 2 factors: Earth’s revolution around the Sun during the time span of 365.25 days Tilted axis of approximately 23.5° The amount of solar energy Earth’s northern and southern hemispheres receive at different times of the year continuously changes during a revolution.
Seasons on Earth Winter Solstice December 21/22 SDR 23.5° S (Tropic of Capricorn) 10 hrs. day, 14 hrs. night Spring (Vernal) Equinox March 20/21 SDR Equator 12 hrs. day, 12 hrs. night Summer Solstice June 21/22 SDR 23.5 °N (Tropic of Cancer) 14 hrs. day, 10 hrs. night Fall (Autumnal) Equinox September 22/23 SDR Equator 12 hrs. day, 12 hrs. night
Seasons on Earth There is only a varying angle of incidence of the Sun’s rays from season to season. We receive more energy from the Sun when it’s shining on Earth’s surface at a steeper angle of incidence.