by Pat Browne
People often ask if stars and clusters are more easy to observe in the Winter in the Northern Hemisphere because of the cold air mass.
Actually the winter sky in the Northern Hemisphere happens to contain a number of outstandingly large, highly luminous stars, and this is the major reason why our winter sky naturally appears more brilliant from the light of distant suns.
These stars are brighter both intrinsically (many of them are classified as “Giants” in the stellar classification) or because their distance to our solar system is closer (which makes them appear visually brighter). Take for instance the three top stars in the constellation Orion: Betelgeuse, Meissa and Bellatrix . Their properties are of Giant type and literally dwarf the sun in terms of either Mass, equatorial Diameter, or Luminosity.
Types of Bright Stars in Orion
Betelgeuse – Ib (Supergiant)
- Ninth brightest star in the night sky and second-brightest in the constellation of Orion. Betelgeuse would be the brightest star in the sky if the human eye could view all wavelengths of radiation.The star is classified as a red supergiant and is one of the largest and most luminous stars visible to the naked eye. If Betelgeuse were at the center of the solar system, its surface would extend past the asteroid belt, wholly engulfing the orbits of Mercury, Venus, Earth and Mars. Calculations of its mass range from slightly under ten to a little over twenty times that of the Sun
Meissa – III(Bright Giant)
- A giant star with a stellar classification of O8 III . It is an enormous star with about 28 times the mass of the Sun and 10 times the Sun’s radius. The outer atmosphere has an effective temperature of around 35,000 K, giving it the characteristic blue glow of a hot O-type star.
Bellatrix – III (Bright Giant) 5° west of the red giant Betelgeuese .
- Bellatrix is a massive star with about 8.6 times the Sun’s mass. It has an estimated age of approximately 25 million years; old enough for a star of this mass to consume the hydrogen at its core and begin to evolve away from the main sequence into a giant star. The effective temperature of the outer envelope of this star is 22000 K, which is considerably hotter than the 5,778 K on the Sun. This high temperature gives this star the blue-white hue that occurs with B-type stars.
Starlight – The Signature of Stars “hidden in light”
“No laboratory jar on Earth holds a sample labeled ‘star stuff’ and no instrument has probed inside a star. The stars are beyond our reach, and only information we can obtain about them comes to us hidden in light” – Michael A. Seeds , Horizons – Exploring the Universe
Star Colors Determine Star Temperature
Stars show different colors because of temperature differences; some are hotter than others. Deep in their interior all stars are enormously hot (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.
You will notice this when you look at the stars in the constellation Orion. You will also notice a tremendous variation in brightness. Astronomers use the term “Luminosity” to measure brightness. In astronomy, luminosity is the total amount of energy emitted by a star, galaxy, or other astronomical object per unit time. It is related to the brightness, which is the luminosity of an object in a given spectral region.
What we usually measure from a large object like a star is the energy flux, the power emitted per square meter. The word flux means “flow” here: we are interested in the flow of power into an area (like the area of a telescope mirror). It turns out that the energy flux from a star at temperature T is proportional to the fourth power of its absolute temperature. This relationship is known as the Stefan-Boltzmann law and can be written in the form of an equation as:
where F is the energy Flux and σ (Greek letter sigma) is a constant number. Notice how impressive this result is. Increasing the temperature of a star would have a tremendous effect on the power it radiates. If the Sun, for example, were twice as hot—that is, if it had a temperature of 11,600 K—it would radiate 2 4 , or 16 times more power than it does now. Tripling the temperature would raise the power output 81 times. Hot stars really shine away a tremendous amount of energy. What powers the stars including our sun? Nuclear Fusion! For more general information on the stellar properties of stars see: Night Sky Article Stars in our Galaxy
- Betelgeuse, Meissa, Bellatrix have Giant Luminosity Class designations as shown schematically in the H-R diagram below:
Learning to Star Hop – Simple Example
Since we have such bright stars in our winter sky, we can use them as intermediate targets on our way to more distant, more difficult, celestial targets. Such objects like star clusters and galaxies can only be seen with the aid of binoculars or a telescope, capable of concentrating more starlight into our eyes.
We use the Star Hopping technique to place our light gathering instruments in range of our target. We start with the bright naked eye stars so that we can place our finder in a known patch of sky. To start the starhop, we check the, map and then check the view in the finder scope or binoculars to compare the image of the map to the image in the eyepiece. The next bit consists of successive leaps from a star that is visible either with the naked eye or with a finder-scope, to another star and so forth until the aimed target is reached. We learn this technique at our Night Sky Sessions. We use Stellarium to plan starhops. Stellarium http://stellarium.org/en_CA/ will give you a good idea of the location of the constellation you wish to starhop in and will help you to decide an appropriate time for viewing .
Here is a beginner starhop exercise around the top of the constellation Orion.
The circles drawn on the Stellarium map mark a 4 degree field . You could use this with a finder scope or binoculars.
Starhop to Stellar Nebulae in Orion
Once you get comfortable with slowly hopping from one target to the next you can go for the fainter ‘fuzzies’ in Orion, namely the two stellar nurseries – The great Orion Nebula Observing Orion Millstone Night Sky article and a much fainter luminous source M78.
This starhop starts off on Rigel (1), the brightest star in Constellation (when not outshone by the variable Betelgeuse) a blue-white supergiant (compare this to Betelgeuse) It estimated to be roughly 200000 times as luminous as the sun. It has exhausted its core hydrogen and swollen out to roughly 100 times the Sun’s radius.
Our starhop continues eastward into the heart of the M42 at hop (5) wanders through another patch of nebulousity until it lights onto the field of M78 (7)
