CHAPTER 2

THE SKY

THE STARS

Since ancient times, people have named groups of the stars for heroes, gods and mythical beasts.

These groups of stars are called constellations.

Different cultures named groupings of stars different names but every star is a member of one and only one constellation.

Asterisms

While there are 88 official constellations, the sky also contains asterisms, which are star groupings that are more loosely defined.

The Big Dipper is an asterism that is part of the constellation Ursa Major (The Great Bear).

Constellations and asterisms aren’t necessarily closely associated with one another, they just lie in approximately the same direction from Earth.

The Names of Stars

Most constellation names come from Greek translated into Latin from the fall of Rome to the 19th century. Most are named for mythological creatures and heroes.

However, most star names come from ancient Arabic.

Betelgeuse, the bright red star in Orion comes from the Arabic yad al jawza, which means the armpit of Jawza (Orion).

Types of Stars Variable stars

RR Lyrae

Cepheid variables

Red giants

Brown dwarfs

White dwarfs

Supergiants

Neutron stars

Variable stars

Variable stars are stars that vary in brightness over time. These changes usually occur very slowly, over a period of months or years, but sometimes it can take just hours.

Some types of variable stars are red giants, eclipsing binaries, RR Lyrae and cepheid variables.

Eclipsing binary star

A binary star is a double star system in which two stars orbit one another around a central point of gravity.

An eclipsing binary occurs when the plane of a binary’s orbit is nearly edgewise to our line of sight.

Sirius α and β are binary stars.

RR Lyrae

RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances.

Because of their age, they are relatively dim.

Cepheid variables

Cepheid stars oscillate between two states: In one of the states, the star is compact where large temperatures and pressure gradients build up in the star. When the star is in its expanded state, the pressure weakens and the star contracts back to a compressed state.

Cepheid variable stars have masses 5-20 times the mass of our star. The more massive stars are luminous with extended envelopes that are outer layers of gas.

Cepheid variables as seenfrom Hubble telescope

Red Giants!

The most common of the variable stars, red giants are stars of average size, similar to our sun, in the final stages of life.

In the last several million years of its life, the star will become alternately brighter and dimmer, spending about a year in each phase until it eventually runs out of fuel.

Brown dwarfs

Brown dwarfs are objects which have a size between that of a giant planet like Jupiter and that of a small star.

It is thought they are made of gas and dust but lack the size it takes to produce enough pressure to begin nuclear fusion. In theory, an object with less than 8% of the mass of the sun cannot become a star.

So why would we care about brown dwarfs? It is possible that a great deal of the mass in the universe is in the form of brown dwarfs, and since they do not give off much light, they could constitute part of the "missing mass" problem faced by cosmology.

White dwarfs

White dwarfs are formed from stars about the size of our sun when their hydrogen is used up.

In the absence of fusion, gravity takes over and causes the star to collapse upon itself.

The bigger the original star, the smaller the white dwarf it becomes. Due to the strong gravitational field, the larger stars collapse more completely than smaller stars.

About 1,600 light-years away, two dense white dwarfs in the J0806 binary star system orbit each other once every 321 seconds. When they reach the end of their long evolutions, smaller starstypically become white dwarfs.

Supergiants

Supergiants represent the final stages of life of only the most massive stars and hence are quite rare.

It’s believed that at the end of a supergiant’s life, there is a supernova which then creates a neutron star or a black hole.

Blue Supergiants Blue supergiants are rare but burn bright

and hot at surface temperatures of 20,000-50,000o C

Blue supergiants represent a slower burning phase in the death of a star.

Rigel, the brightest star in the Orion constellation, is a blue supergiant

You can catch Orion in the east before dawn in late summer, but on January evenings Orion is riding at its highest in the mid-evening sky. Look for Orion high in the south on these Northern Hemisphere winter evenings. By early March, Orion – with blue-white Rigel in its midst – is high in the south as soon as the sun sets. By early May, it is setting before the sky has a chance to get really dark.

Red Supergiants

The red supergiant is a star that is bigger and more massive than the red giant. Red supergiants usually have a diameter which would be several hundred times that of the Sun and they would weigh 10 times more than the Sun.

The red supergiant phase is very short lived, they only last for a hundred to a million years before exploding into a supernova. Although red supergiants are very rare, clusters of them do exist in space.

More than two dozen stars, appearing yellow near the center of this Spitzer Space Telescope image of the cluster RSGC2 are red supergiants. Warm dust in the region glows red. The blue oval with a pink outline attop left may be the result of an ancient supernova and the larger blue patch below center is a region of current star formation. Image credit: B. Davies/RIT/NASA

Neutron stars A neutron star is a type of

stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event.

In Chandra's image (right), the colors of red, green, and blue are mapped to low, medium, and high-energy X-rays. At the center, the bright blue dot is likely the neutron star that astronomers believe formed when the star exploded.

Nova

A nova occurs when one member of a binary star system temporarily becomes brighter. Most often the brighter star is a shrunken white dwarf and its partner is a large star, such as a red giant.

From time to time (50 years or so), matter is transferred from the larger star to its smaller partner, initiating a nuclear chain reaction on the smaller star’s surface.

When the reaction ceases, the material blows off the star, causing it to glow brightly. Days or weeks later the star fades and the process begins again.

Supernovas

This is the aftermath of supernova 1987A – a shock wave of material unleashed in the blast slammed into a ring of debris likely shed by the star 20,000 years previously. Image: NASA, ESA, P. Challis and R. Kirshner (Harvard-Smithsonian Center for Astrophysics)

How to find the stars! In 1603, Bavarian lawyer

Johann Bayer published an atlas of the sky called Uranometria. He assigned lower-case Greek letters to the brighter stars of each constellation.

The brightest star is usually designated α (alpha), the second brightest β (beta), γ (gamma) and so on.

Sirius, or Isis, is an alpha star and the brightest we can see.

Sirius α is about 8.5 ly away and heading toward Earth at many 1000’s of miles per second. Its annual heliacal rising exactly matches Earth’s solar year at 365.25 days.

Ancient Egyptians knew this and marked the rising of Isis with the start of their calendar year.

New Year’s marks the return of Sirius to the midheaven position at midnight.

Sirius B is a white dwarf star. Sirius β spins on its axis 23 times

per minute, creating an immense magnetic field.

Favorite stars Construct a chart to describe these stars.

Your chart must include:

Betelgeuse Polaris Sirius Rigel Aldebaran Spica Vega Alpha Centuri

Translation of name Age of star Type of star Size of star relative to our sun Magnitude of star Constellation Distance from Earth When you can best see it

from Grand Junction. Star speed of rotation/Time it

takes to rotate

Star ChartBetelgeuse

“armpit of Orion” 7x106 years old Red super giant Size: 700-900 x the size

of the sun Magnitude: 0.42 Constellation: Orion Distance: 642 ly Best seen: Jan-mid March Speed of rotation: 5 km/s

Star ChartPolaris

“The Bear’s Tail” Age: 7 x10 7 years Super Giant Size: 46 x the size of

Earth Magnitude: 1.985 Constellation: Ursa minor Distance from Earth: 434

light years Best seen: year round Speed of rotation: 17

km/s Rotates in 19 hours

Star ChartSirius

“Glowing or Scorcher” “Dog Star”

Age: 4-6 million years Type: White dwarf, blue

in color Size: 2 x larger than our

sun Magnitude: -1.46 Constellation: Canis

major Distance: 860 light years Best seen: winter and

spring Speed of rotation: 16

km/s

Star ChartRigel

“Central one” or “Right foot”

Age: < 3.4 x 109 years Type: Blue super giant Size: 60 x larger Magnitude: .12 Constellation: Orion Distance from Earth:

77.28 light years Best seen: Jan-mid March Speed of rotation: 40 km/s 6th Brightest star—40,000

x brighter than our sun

Star ChartAldebaran

“The follower” Age: 10 million Type of star: Orange giant Size: 44 x bigger than our

sun Magnitude: 0.87 Constellation: Taurus Distance from Earth: 65 light

years Best seen: summer Rotation: 5.2 km/s Rotates in 643 days The fiery eye of the bull, it

burns 153 x brighter than our sun

Star ChartSpica

“Ear of corn” Age: 275 million years old Type of star: blue dwarf Size: 7-10 x larger than our

sun Magnitude: 0.98 Constellation: Virgo Distance from Earth: 275 ly

or 80 parsecs Best seen: spring to late

summer Speed of rotation: 199 km/s It is a whirling binary star. 15th brightest star from

Earth

Star ChartVega

Name: Swooping eagle/falling

Age of star: 3.4 x 108 years

Type of star: Variable white dwarf

Size: 2x the size of our sun Magnitude: 0.03 Constellation: Lyra Distance from Earth: 25 ly Best seen: summer Speed of rotation: 236

km/s Time of rotation: 12.5 hrs

Star ChartAlpha Centuri

Name: Brightest star in Centaurus

Age: 5-6 x 109 years Type of star: Yellow star Size: 110% the size of our

sun Magnitude: -0.27 Constellation: Centaur Distance: 4.367 ly Best seen: only in southern

regions of U.S. Speed of rotation: 2.7 km/s 4th brightest star Closest star to Earth

Extreme Stars (46:00)

Embryonic Stars Emerge from Interstellar "Eggs““Evaporating Gaseous Globules”

The Brightness of Stars Ancient astronomers divided the

stars into six magnitudes with the brightest called first-magnitude stars and the least visible sixth-magnitude stars.

Modern astronomers have a magnitude scale that starts with the brightest stars with negative numbers. This is because Hubble can detect stars as faint as 30th magnitude so the scale was readjusted.

Sirius, the brightest star in the sky, has a magnitude of -1.44, the sun is -26.5 and the moon is -12.5.

These numbers are apparent visual magnitudes that describe how the stars look to human eyes observing from Earth.

Magnitude and Intensity

Because light brightness is subject to the physiology of the eye and psychology of perception, an accurate reference to starlight is flux. It is the measure of the light from a star that hits 1 square meter in 1 minute. This measurement is directly related to the intensity of the light.

Star brightness is determined by comparing the intensities of two stars, Ia and Ib by calculating their ratios:

Ia ÷ Ib

For further details on how to calculate modern magnitudes, please refer to page 17 of the textbook.

Precession(7 minutes)

Celestial sphere

The celestial sphere seems to rotate the stars, moon and sun in westward around Earth.

You can only see the area above the horizon from any place on Earth.

The celestial equator lies between the north and south celestial poles with the east-west points in between.

The zenith marks the top of the sky above your head and the nadir marks the bottom of the sky underneath your feet.

Measuring distance Astronomers measure distance

across the sky as angular distances in degrees, minutes and seconds of arc.

A minute of arc is 1.60th of a degree and a second of arc is 1/60th of a minute of arc.

Angular diameter is the angular distance from one edge to another.

Circumpolar constellations never rise or set. These include:

Ursa Major, the Big Bear (includes the Big Dipper)

Ursa Minor, the Little Bear Cassiopeia, the Queen of

Ethiopia Cepheus, the King of Ethiopia Draco the Dragon

The Cycles of the Sun

Secrets of the Sun (53:06)

The Seasons

The seasons are not caused by any variation in the distance from Earth to the sun. What are they caused by?

They are caused by the changes in the solar energy that Earth’s northern and southern hemispheres receive at different times of the year.

The two equinoxes mark the beginning of what seasons?

Vernal equinox is the beginning of Spring and the autumnal equinox is the beginning of Fall.

What two seasons are marked by the solstices? Winter and Summer

Earth’s light and the solstices

Earth’s lighting and the summer solstice

Earth’s lighting and thewinter solstice

Passive solar design

Ecliptic

The ecliptic is the apparent path of the sun in its yearly motion around the sky.

It is a projection of Earth’s orbit on the sky.

You can also define it as extending the plane of Earth’s orbit out to touch the celestial sphere.

The Zodiac, Astrology and Pseudoscience

The Zodiac is a band 18” wide centered on the ecliptic; a “highway” that the planets follow. Divided into 12 segments, they are named for the constellations and represent the signs of the zodiac.

Horoscopes show the location of the sun, moon and planets among the zodiacal signs with respect to the horizon at the moment of a person’s birth as seen from that longitude and latitude.

Horoscopes are specific to an individual unlike the ones you may read in the newspapers.

Astrology is a pseudoscience that depends on belief rather than evidence as true science does.

Milankovitch hypothesis In 1920, Yugoslavian meteorologist Milutin

Milankovitch proposed that changes in the shape of Earth’s orbit, precession and inclination affect Earth’s climate and trigger ice ages.

The elliptical shape of Earth’s orbit varies every 100,000 years. Currently, we are 1.7 % closer than average to the sun during northern hemisphere winters and 1.7 % farther away in northern hemisphere summers, making the northern climate slightly less extreme.

Second factor is precession which causes Earth’s axis to sweep around a cone with a period of about 26,000 years which changes the location of the seasons around Earth’s orbit. In 13,000 years, northern summers will occur on the side of Earth’s orbit where it is slightly closer to the sun and summers will be warmer.

Third factor is the inclination of Earth’s equator that varies from 22o to 24o with a period of about 41,000 years. When the inclination is greater, seasons are more severe.