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ASTR 313, Majewski [FALL 2002]. Lecture Notes

ASTR 313 (Majewski) Lecture Notes


COMETS

REFERENCE: Kaler Chapter 12.

Comet Hale-Bopp.
Comets are chunks of ice and rock ("dirty snowballs") and tend to live most of their lives at very large radii from the Sun.

The Solar System contains billions of comet nuclei, but most are beyond the orbit of Neptune.

  • Most have very elongated orbits and reach small distances from the Sun only infrequently.

  • These comets are new comets when they are seen for the first time in the inner solar system.

  • Some new comets, like Hale-Bopp a few years ago, can be spectacular.

Some comets, however, get gravitationally captured by Jupiter into orbits with shorter periods (< 100 years); these are called periodic comets.

  • Periodic comets, like Halley's Comet, come at reasonably well defined intervals.

  • The name is a misnomer -- periodic comets are really comets with periods short enough that humans see repeated visits of them.

  • About 900 comets have been observed well enough that their orbits can be calculated.

Most comets become visible only when they get close enough to the Sun (several Earth radii) that the ice is melted (sublimated) directly into gas.

  • The source of the gas and released dust is a few km diameter nucleus.
  • The gas produce a bright spherical coma. Comets a re generally discovered when the coma begins to develop and they look like small nebula -- but nebula that change positions from night to night.

  • Sometimes the gas produces a tail -- gas tails tend to glow blue and that always point away from the Sun since plasma ions in the tail become entrained in the solar wind.

  • When the gas sublimates, trapped dust is released and can form a dust tail, which tends to be red. Dust tails may be fan-shaped as the particles get pushed by solar radiation into larger orbits that have longer periods (Kepler's Third Law!). The dust tails more or less lag behind the comet and trace the general orbit of the comet.

Comet observing can be fun because the shape and brightness of the coma and tails can take on different shapes over hours, as the dirty snowball exposes different sides to the Sun and different pasts sublimate.

  • Note that the position of the comet changes from night to night as well.

  • Of course, comets tend to become brightest as they approach the Sun, so many comets are best just before dawn in the East or after dusk in the West.

  • Binoculars tend to provide the best view, because the field of view is large enough to see comma and tail.

  • Sweeping the expected position in the sky with binoculars is the best way to find fainter comets, and use averted vision!



METEOR SHOWERS


Meteors


Also known by the misnomer "shooting stars" -- of course these are not stars!

Caused by solid pieces of interplanetary debris entering the Earth's atmosphere and vaporizing.

Note terms:

  • Meteoroid is term for the debris while in space.

  • Meteor is term for the luminous phenomenon during vaporization.

  • If the particle survives to fall onto the surface of the Earth, the remains are called a meteorite.

  • A fireball is an exceptionally bright meteor that often leaves a lasting, luminous vapor trail behind.

  • A bolide is a fireball that breaks up or explodes in mid-air, often with an audible explosion.

About 25 million visible meteors fall on the Earth per day.

  • Humans do not see a large number of them because a meteor must be within 150-200 km to be seen.

  • Many places of the Earth uninhabited.

  • Most meteors burn up between 80-130 km above Earth.

  • More meteors are visible between midnight and dawn than any other time.

Why we tend to see more meteors after midnight:

  • Imaging solar system meteoroid debris consists of randomly moving particles with a top speed of about 42 km/s.

  • After midnight, we are on the side of the Earth that is plowing into the meteoroids. Therefore:

    • Meteoroids moving towards the Earth fall into Earth atmosphere.

    • Meteoroids moving away from Earth more slowly than the Earth's orbital velocity (30 km/s) are overtaken and fall into Earth atmosphere.

    • All meteors move through Earth's atmosphere at higher relative velocity because of additional 30 km/s Earth motion -- so these burn brighter and are easier to see.

  • In contrast, before midnight we are on the side of the Earth that is away from the direction of Earth motion (we are on the back of the Earth).

    • The only meteoroids that can get through Earth's atmosphere are those that overtake the Earth -- a small fraction of all meteoroids are moving in same direction as Earth and faster than 30 km/s.

    • The overall relative velocity of the meteor through the Earth's atmosphere will be lower -- so the meteor will be less spectacular and harder to see.



Meteor Showers


Meteor showers occurs when the Earth encounters a swarm of particles moving together through space.

Usually these swarms can be associated with the debris dust trails of comets.

  • Since the orbits of periodic comets are fixed, the debris stream is in a predictable location in the solar system, and on occasion the Earth will cross this location resulting in a predictable meteor shower which always occurs on certain dates of the year.

  • If the comet is relatively young, having made only a small number of visits to the inner solar system, then the debris will be bunched up and orbiting near the comet. In this case the showers may only be good if the Earth and the debris stream intersect at the same time at the orbit crossing location.

    • The Leonid meteor shower is an example of this type of meteor shower -- the debris is clumped up and orbiting with a 33 year period, resulting in spectacular showers every 33 years.

  • An older debris stream will be more spread out along the orbit resulting in similar shower intensity every year.

  • Most well-known meteor showers can be associated with specific comets.

  • The radiant of the meteor shower is that part of the sky from which all meteors appear to diverge. It is the direction of space from which the debris is coming.

    • The names of meteor showers are taken from the constellation in which the radiant lies.


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All other material copyright © 2002 Steven R. Majewski. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 313 at the University of Virginia.