at night travel great distances - loudoun county public ... · parallax stars that are close to...
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� The light from the stars we see
at night travel great distances
to reach Earth.
� Other than the sun, Proxima
Centauri is the next closest
star. star.
� The light that we see from it
takes over 4 years to reach our
eyes.
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� Other light from
stars has taken
thousands,
millions, or even millions, or even
billions of years to
reach Earth.
The stars of Orion's belt: Mintaka, Alnilam and
Alnitak. (800-1300 ly)
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� When ancient people
looked at the sky, they saw
pictures of mythic heroes
(Hercules), animals (Draco)
and objects (Lyra).
� They used to tell stories � They used to tell stories
about the groups of stars
in the sky, which are now
called constellations.
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� 88 different
constellations can
be seen from the
Earth’s northern &
southern
hemispheres.
� Constellations are � Constellations are
not natural; they are
man’s grouping of
stars as we view
them from Earth.
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� The Big Dipper is
probably the best
known asterism – a
small grouping of stars
from a larger from a larger
constellation.
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� The Big Dipper
is part of the
constellation constellation
Ursa Major or
the Great Bear.
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The dipper can be used to find other
constellations.
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� Draw a line through the two stars farthest from the
dipper’s handle.
� The line through these “pointer stars” leads to the
last star in the handle of the Little Dipper.
� This is Polaris, or the North Star.
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The movement of the stars in the sky is due the Earth’s rotation and revolution.
1. The Earth rotates from
West to East (rule of
thumb)thumb)
� So the whole sky
appears to move from
East to West.
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The sun and the stars and the moon rise in the
east & set in the west.
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2. One star seems to
be stationary – the
North Star called
Polaris.
This is because it lies This is because it lies
directly above the
North Pole.
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All other stars appear to revolve around it.
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3. Stars or constellations
close to the North Star
can be seen all year.
They are called
circumpolar stars or
constellations.
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4. A constellation’s position in the sky changes
with the seasons.
Some constellations can only be seen during
certain seasons.
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During the winter
months, Orion can
be seen from the
Northern
Hemisphere.
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• But six months later, the Earth has moved to
the other side of the sun.
• Orion cannot be seen because it is facing the
daytime side of the Earth.
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Now people on Earth can see Lyra, a summer
constellation.
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Summer - The Teapot, part of Sagittarius, crosses the night sky during the summer. When we look at the Teapot, we are looking towards the center of our Milky Way galaxy
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Fall - Pegasus and Andromeda fly near Cassiopeia the Queen. Looking south of and far beyond the Milky Way, we see the distant Andromeda galaxy
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Winter - Looking out through the spiral arms of the Milky Way galaxy we see Orion the Hunter chasing Taurus the Bull.
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Spring - When Leo the Lion crosses the sky at night, we see galaxies north of our Milky Way galaxy, like the group called the Leo Triplet
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Ursa Minor – the little dipper
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Distances in Space
� Units we use to measure distances on Earth are way too small for the extremely large distances in space.
� The distance between the Earth and the Sun for example is 150 million kilometers or 93 for example is 150 million kilometers or 93 million miles.
Now that’s a BIG number!
Here’s something easier to remember…
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1 AU (Astronomical Unit)
= 93 million miles
= the distance from the Earth to the
Sun
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It is easy to measure distances
in our solar system with AU’s,
but what about for far away
stars and galaxies?
� Our closest star, Proxima Centauri is… � Our closest star, Proxima Centauri is…
24.8 trillion miles or 260,000 AU away
from Earth!!
Now that’s another BIG number!
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☺ let’s make it easy again…
� Proxima Centauri is 4.2 light years
away
� What is a light year?� What is a light year?
� A unit that measures the distancelight travels in one year
� Light travels 9.5 trillion kilometers in
one year (5.8 trillion miles)
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Light travels 300,000 km/sec so…
� Sunlight reaches Earth in 8 minutes
� Sunlight reaches � Sunlight reaches Pluto: 4 hours
� Light from ProximaCentauri reaches Earth: 4.2 years
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How far away is Proxima Centauri?
� 24.8 trillion miles or 260,000 AU or 4.2 light years
� It is much simpler to speak & write in light-years!!
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How do astronomers measure the distance to stars?
� One method astronomers use to measure distances to stars is called parallax.
You try it…You try it…
� If you hold your finger in front of your face and close one eye and look with the other, then switch eyes, you'll see your finger seem to "shift " with respect to more distant objects behind it.
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Parallax
� Stars that are close to Earth show parallax as Earth moves from one side of the sun to side of the sun to the other.
� Astronomers can then determine the distance using geometry!
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� Luminosity is another word for the
brightness of a star.
� Ex: different watt light bulbs.
sz� A star’s luminosity depends on size
and .
� The BIGGER the star - the brighter the star.
� The hotter the star - the brighter the star.
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When you look up at the stars at night, it is obvious that some are brighter than others.
� However this is deceptive since the observed brightness of a star depends on how far away you are from it.
Astronomers express luminosity of a star through a Astronomers express luminosity of a star through a scale of magnitudes.
is the measure of the brightness of a star.
� Every star needs 2 different magnitudes Magnitude and Magnitude.
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� How bright a star appears to us from Earth(comparing which star looks brighter in the night sky)
� We still use the Greek system of apparent .
� The brightest stars were said to be 1st
magnitude.
� The dimmest stars visible in a dark sky far from a city are 6th magnitude. � The dimmest stars visible in a city are often only 3rd
magnitude.
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� Later, it became necessary to assign to some stars a magnitude brighter than 1.0.
� Ex: Vega has magnitude of 0 and Sirius has a magnitude -1.4, the sun has a magnitude of -26.74.
� So, the more negative the value of apparent magnitude, the brighter the star appears. magnitude, the brighter the star appears.
� Conversely, the larger the value of magnitude (more positive) the fainter the star appears.
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� How bright a star would be if all stars were the
same distance from Earth (32.5 light years).
� The star that is actually brighter comparing
them side-by-side.
� The absolute magnitude of the Sun as about � The absolute magnitude of the Sun as about
+5.
� The absolute magnitude of Rigel is about -7,
meaning it is really incredibly bright.
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ColorColorColorColor Surface Temperature (C)Surface Temperature (C)Surface Temperature (C)Surface Temperature (C) ExamplesExamplesExamplesExamples
Blue over 25,000 10 Lacertae
Blue 11,000 - 25,000 RigelSpica
Blue 7,500 - 11,000 Sirius, Vega
Blue to White 6,000 - 7,500 Canopus, Procyon
White to Yellow 5,000 - 6,000 Sun, Capella
Orange to Red 3,500 - 5,000 Arcturus, Aldebaran
Red under 3,500 Betelgeuse, Antares
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A light echo around the star V838 Monoceros.A light echo around the star V838 Monoceros.A light echo around the star V838 Monoceros.A light echo around the star V838 Monoceros.•Not quite yet understood. Not quite yet understood. Not quite yet understood. Not quite yet understood.
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Red Giants
High LowTemperature
The H-R diagram plots surface temperatures of stars against
their absolute magnitudes.
White dwarfs
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Main Sequence Stars
� Where a star begins its life (fusion of H to He)
� Majority of all stars lie in a band that runs diagonally through the
diagram
� Theme: hotter the temp – brighter the star
Giants and Super GiantsGiants and Super Giants
� A group of cool stars that are very bright because of the large size
� Found in the upper right corner of the diagram
White Dwarfs
� Near the end of their lives
� A group of hot stars that are dim because of their small size
� Found in the lower left corner of the H-R diagram