NightSky Lecture 2

Lecture 2 Stars in space in our Milky Way Galaxy

Spring 2013 Astronomy Course

Mississippi Valley NightSky Conservation

Program developed by:

The Night Sky Around Us

The Local Night Sky as it appears May 10 2013 at 10PM EDT –
courtesy Earth Centered Universe

II Stars in our Milky Way Galaxy

… the vast expanse of interstellar space, galaxies, suns and planets in their courses…

WHERE

• Locating stars on the Celestial Sphere

• Star Maps and Constellations, Planisphere or Monthly Sky Map

WHEN

• Do they rise and set on our local horizon

• Star time (When do the same stars appear to rise night after night)

WHAT

• Constellations and Bright Star
  Identification

• Stellar properties – Colour,Temperature,Mass,Luminousity,Magnitude

• Stellar Classification: OBAFGKM Sequence derived from stellar spectra

WHO

• Pioneers in stellar astronomy:

• Annie Jump Canon

• Ejnar Hertzprung- Henry Russell

Objects on the Celestial Sphere – since last week have shifted to the West…

Last week our planisphere for May 3 showed constellation Leo just West of the Meridian, our
North-South line on which the stars culminate (reach highest elevation) when they are due South.

Now notice where Leo is…. It is starting to set in the West

Recall: What we see in the sky depends on

• Day in our orbit
• Our latitude for our local horizon
  given that the altitude of the Pole Star is the same as our latitude

• Time … Sidereal or Star Time

Sidereal Time = A Day On Earth using Star Time

Sidereal Time = Hour and Day with respect to the Stars..(not the Sun)

• 1 Day = 1/365^th of a circle about one degree
  around the Sun.
• Earth rotates on its axis as well as rotating around the sun.
• So, the time for a star to return to the same place in our sky the following evening is only 23 hours, 56 minutes and 4 seconds (not 24 hours)

This is called a Sidereal Day. It measures both Earth Rotation and Orbit with
respect to the stars

Understanding Star Time – Interval between Sidereal Days

• Point A on the second day is still looking at a parallel
  direction in space .
• However the star will appear at that spot in the sky 4
  minutes of earth time earlier.
• This corresponds to the amount of rotation taken up by
  1440 (minutes/day) as the fraction 1/365
  (orbit/day) So 1440/365 is roughly 4 minutes (shorter).

Spin around as we revolve… to reach the same position toward the stars at a ‘fixed’ infinite distance…

…then we spin around a bit further towards the center of our orbit … towards the sun.

‘We’re captive on a carousel of time. We can’t
return we can only look behind from where we came … and go
’round and ’round and ’round in the circle
game...’ J. Mitchell

• The stars rise 4 minutes earlier each day because the earth has also moved through its orbit as it has rotated around from night to day to night

Leo is an early Spring constellation and will rise and set earlier each
night . In later spring, these stars will eventually set while it is still daylight.
Constellation Leo will be followed by the summer constellation of Lyra
(which contains the bright star Vega).

Celestial Objects rise in the East and set in the West

Apparent Motion of the Sky

Every night, stars rise and set… just as every day, the Sun, our star rises and sets… Because the Earth is turning and
carrying the observer from West to East, the stars seem to be moving from East to West. What we see in the sky changes all the time because the earth is always rotating.
The Sun in the morning appears to rise in the East, At noon, it appears in the South (for our latitude) part of the sky. In the evening it is seen setting in the West.

• Similarly, you can see a bright star rise above the eastern
  horizon in the early part of the night. At about midnight (middle of
  the night ~ high noon for day) the star culminates. (rises highest)
  in the southern part of the sky. When the night ends, and morning is
  coming, the star will be setting in the western part of the sky.
• The motion of the sky is called the apparent motion of the
  sky because it is really we on the surface of the earth who are
  rotating in this way.Courtesy Leo Enright, Sky Motion, The Beginner’s
  Observing Guide

Star Positions throughout the Evening for a given Latitude

• Star A is could be a star in the
  Big Dipper

• Star B and C will eventually set
  below the horizon.

• Some Objects never rise above our
  Local Horizon as in (a)…It all
  depends on where you, the
  Observer are standing on the earth.

Using Star Maps

If you are not using the adjustable planisphere or a planetarium program, and just want to observe with star maps, you will need to use more than one over the course of the evening . Leo Enright has provided 6 star maps in The Beginning Observer’s Guide thatshow the sky for two months as a time. The one we are using is Map 3
(May and June).

Map Orientation

• The heavy black circle is the horizon, the line where the earth meets the sky.  Place the map in the orientation you are facing. For northern hemisphere observers, we face South to see stars culminate; so as with the planisphere, place South at the bottom when you are facing South. This will establish the directions to the other constellations for this time of year.

What Visual Observations tell us about Stellar Properties

The Bright Stars and Asterisms in the Sky:

Many observers ask questions about the brighter stars they see night after night. The nature of stellar radiation exhibiting the following properties helps us to understand how astronomers classify stars.

1. Colour: Observe that stars have colour: Vega, we observed was bluish-white.
   This determines a spectral temperature classification.
2. Brightness: Observe both Vega and Regulus are bright stars
   meaning they have a certain visual magnitude classification.
3. Variable radiation:Certain stars vary in brightness over the course of a night(short period variables)
   Special variable stars called Cepheid variables show a periodic variation where the period tells us the intrinsic luminousity and hence calibrates
   a measurement to these stars
4. Multiplicity:In spite of their seemingly random distribution on the celestial sphere, the stars are highly organized. They prefer to form doubles (called binairies)
   and multiple systems . These stars are of immense value because we can plot their orbital period and then apply Keplar/Newtonian formula to determine their masses

Magnitudes of Stars – How Bright is your Star: first to sixth magnitude

The Greeks divided the stars into 6 brightness categories. The brightest stars were called category one or first magnitude. The dimmest stars were category 6 or sixth magnitude. The lower, the number the brighter the star. This system of first-sixth magnitude brightness categories is still used today for visual observing. However the scale has been greatly extended to include bright satellites, as well as extremely dim objects which can be detected by CCD cameras in dedicated observatories. The seven stars in the Big Dipper asterism have a visual magnitude < 2. Vega, the brightest star in the Summer Triangle asterism is magnitude ZERO!

Brightness vs. Distance of an object

We now recognize why stars are brighter or fainter.  A star might be brighter than another star  because:

1. It is extremely energetic and its stellar properties produce intense visible radiation. In this case
   the objects intrinsic brightness or luminousity dominates.

2. It is at a closer distance to our solar system

Examples of Stellar Properties:

• Magnitude (apparent , absolute (calibrated for estimated
  distance))
• Spectral Type (indicator of colour and temperature chemical
  composition)
• Variable (varies in brightness often periodically)
• Part of a binary or multiple star System

Star Colours used as indicators of Stellar Spectra – the Key to Stellar Properties

The main reason why stars are differently colored is that some are hotter than others. Deep in their interior all stars are enormously hot (measured in millions of degrees), but their temperature lessens towards their outer layers, and the coolest star pours out most of their visible radiation in the red part of the spectrum. Hotter stars like the Sun appear yellow, still hotter stars appear white, and the hottest appear blue. Stars radiate light a little like glowing coals in a campfire. Just as a glowing red-hot coal is cooler than a white-hot coal, for example, so a red star is cooler than a white star, and
a white star is cooler than a blue star.

Star Color and Temperatures

Star Color and Spectra

Stellar Classification: Colour and Star Spectral Components of the Light

In the late 19th century, Harvard astronomers developed a system to classify stars not directly according to color,
but by the peak of spectral radiation and dark lines showing how hydrogen gas absorbed light at particular wavelengths.
In other words, they used a prism grating device to photograph the colour components of stellar radiation.
The stellar classification grouping all stars into just 7 categories was born:
OBAFGKM designations are applied to every star. If the stars are main sequence, then the luminousity class is also assigned.
The sun for example is t a G2 star with luminousity class V

Spectral Lines… The fingerprints of stars

The Spectral Sequence …  Temperature and Line Signature

(Click on these images to view the text)

The Hertzsprung-Russell Diagram

Around 1911-1913, the Danish astronomer Ejnar Hertzsprung and American Henry Norris Russell studied star clusters, in which all the stars are roughly the same age, and noticed a clear and surprising relationship between the stars’ brightness and color. They plotted the color and temperature (as represented by OBAFGKM) classification of each star and determined blue stars are brighter and red stars are fainter along the central Main Sequence curve.

Stars are not static eternal creatures, but in a certain sense are living bodies: we speak of stars being born, of evolving and of dying…
courtesy Stars and their Spectra, J. Kaler [p. 29]

• The large majority of stars fall within the Main Sequence which represents normal hydrogen-burning stars arrayed according to
  their masses.
• As the mass of the star goes up, so does the size of its core, so that more fuel is availble for burning.
• More importantly, the interior temperature goes up as well.
• The rate of thermonuclear reaction depends on the mass of the star and hence the luminousity depends on the mass of the star
• The more massive stars burn their fuel much more rapidly, and hence burn out more quickly
• When the nuclear fuel is used up, the core loses a major part of its support, and contracts under gravity: the star then has to heat up which initiates further fusion… more on this later…

For Main Sequence normal mass stars, once a star begins burning hydrogen through nuclear fusion, it settles onto a particular spot on the main sequence and stays there until the hydrogen runs out.
For historical reasons, stars along the main sequence are called “dwarfs” and are given the additional symbol “V”. So the Sun is a G2-type dwarf star, or G2V.
As a star begins to burn helium and heavier elements in the core, it quickly evolves off the main sequence into other types of stars like giants, supergiants, and eventually white dwarfs.