More Points to Ponder – General FAQ from the Nightsky Astronomy Course

by Pat Browne

This question was discussed during our last session while we were touring the Fred Lossing Observatory. The observatory is the place of discovery of 5 comets discovered by local comet hunters R. Meier and D. George , using the  FLO 16″ reflector telescope. http://www.rasc.ca/canadian-comets

Question: What is a comet?

A comet or “cometes” in Greek means ‘hairy’ because comets look like ‘hariy’ stars. Indeed, famous comet hunter David Levy is said to ‘patiently sweep the heavens for those cosmic hairballs called ‘comets'”, – courtesy, Deep Sky Objects, David Levy (Forward by Stephen James O’Meara).

A comet has the following properties:

  • The fuzzy envelope known as the coma, which varies in size with each comet as well as with its distance from the sun. The ability of the comet to form a coma distinguishes it from an asteroid. The coma is usually tens of thousands of kms in diameter.
  • Most comets have a ‘tail’  – often there are 2 parts to that – the gas tail and the the dust tail . The tails of comets are the largest things in the solar system reaching millions of kms in length. Comet tails are formed when the pressure of light from the sun and solar wind drive gases from the core nucleus into the coma and then into the tail. The main tail is thus pointing away from the Sun.  “As the comet approaches the sun, it drags its tail after it, but as the comet leaves the sun’s region, its tail precedes. In other words, the tail then wags the dog.” (courtesy – Helen Sawyer Hogg, The Stars Belong to Everyone.
  •  A comet’s trajectory is a very wide and often one way trip around the solar system. Comets orbits are good examples of ‘conic sections’. They can trace elliptical, parabolic or hyperbolic paths as they round the Sun. Earthlings have recorded periodic ones like Comet Halley, but there are others that we see only once. Their origin is believed to be out past the orbit of Neptune in the Kuiper belt or Oort cloud as shown in this diagram:

cometOrbit

This Spring Comet Panstarrs was a very low altitude comet (for our latitude) near sunset, which prompted me to write an article about the difficulty of getting a clear horizon. You can find out more about this  Stalking a Comet!

Here is a small video of a favourite comet that appeared in the Spring of 2012: Comet Garadd over Miss. Mills Skies

 

This question may have been prompted by the stimulating and intriguing talk by Sanjeev Sivalrurasa, our guest speaker during the Full Moon week. We were talking about tracking the sky versus star trails …

Question:
What kind of orbit is the Hubble Space Telescope in?

Answer:

The HST is in a Low Earth Orbit (LEO) and completes an orbit in just 96 minutes whereas it takes the Earth 24 hours (1,440 minutes) to complete a rotation. The Hubble spacecraft transmits uplinks its data to the TDRSS (Tracking and Data Relay Satellite System) which is in geosynchronous orbit (best place for communications satellites) to be relayed to earth. Since the HST orbits the planet much faster than the Earth rotates, the telescope circles the Earth about 15 times per day and each orbit moves further across the surface of the Earth than the one before. In other words, the HST is never over the same point on the surface of the Earth when it is at the same position in its orbit. This behavior produces a series of orbits whose ground tracks move across the surface of the Earth. see http://www.aerospaceweb.org/question/spacecraft/q0282.shtml

leogeoorbit

 

Question:

Since HST is not on earth, it doesn’t have to compensate for the earth’s rotation; so how does it accurately stay fixed on the stars?

Answer:
Fine Guidance Sensors (FGS) are arguably the single most key pieces of electronics on HST.  The FGS’s are what point HST and keep it locked in position while observations are being made, a difficult task considering that HST orbits the Earth every 96 minutes, and is subject to extreme thermal stresses every time it passes through the solar terminator (the line between darkness and sunlight in orbit). See http://wiki.answers.com/Q/What_is_the_most_important_part_on_the_Hubble_Space_Telescope

Question:
Why do more massive stars live shorter lives?
Answer:

The evolutionary path of  a star depends on the rate of thermonuclear fusion and hence the amount of material it has to burn (i.e. the mass). Our Sun, has an expected lifetime of about 10 billion years. It is now about half way through its life cycle. However, some very
massive stars will live for only a few million years and some low mass stars have been around since the galaxies began to form about 12 to 13 billion years ago. Some of these low mass stars will live for many more billions of years The rate at which fusion reactions occur is a function of both the temperature and pressure in the star’s core. These in turn depend on the mass of the star. It has been found that the rate of fusion is proportional to mass^4.  So, doubling the mass of a star increases the rate of fusion by 16 times.clusterhranim This relationship of Luminosity (which is a direct indicator for rate of fusion) proportional to Mass^4 holds for stars on the Main Sequence, not the Red Giants or White Dwarfs. It is considered one of the ways of measuring ages of clusters in the universe.

 

 

 

 

 

 

mainSequenceAgeImagine that a star population is found where the constituent stars all formed at the same time, but with a wide variety of masses. Initially these objects will begin hydrogen fusion, and all fall along the main sequence. Immediately, however, we can see that this situation will not last – the most luminous stars are short-lived, and will soon disappear from the main sequence, while the less luminous stars will live for a longer (but still finite) length of time. So as time advances, the observed main sequence tends to shorten, like a burning fuse, as the stars expend their hydrogen and enjoy a brief stint as giants before ultimately permanently settle toward the bottom-left corner as white dwarfs. Therefore, the length of the main sequence – or, more specifically, the luminosity and temperature of the “turnoff” point where stars depart from the main sequence to become giants – indicates the age. A star at this turnoff point must be at the end of its lifetime (as all stars more massive than it have already expended their central hydrogen) and therefore its lifetime is its age

see:  http://astro.berkeley.edu/~dperley/univage/univage.html#stellarevolution

 

Question: How do you measure things in the sky?

Answer: Beginners are often at a loss to describe celestial objects positionally . What’s easy to remember is that we are really looking out into our Celestial Sphere.

To understand how we use angles to measure locations in the sky, the Chandra site has an excellent explanation: “The system of angular measurement used by astronomers is based on divisions of the circle. The circle is divided into 360 degrees. Degrees are divided into 60 minutes of arc, or arc minutes, and each minute is divided into 60 arc seconds.” However angular difference between two objects, say between two stars, or between the moon and a planet is our spherical measuring tape around the celestial sphere.

http://chandra.harvard.edu/photo/scale.html

 

angularMeasure

Celestial Sphere Extending from Starship Earth
Celestial Sphere Extending from Starship Earth