Constellations: Lacerta 'the Lizard' Content from the guide to life, the universe and everything

Constellations: Lacerta 'the Lizard'

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
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A lizard.
Precisely the least, the softest, lightest, a lizard's rustling, a breath, a flash, a moment - a little makes the way of the best happiness.
– Friedrich Nietzsche

Name:Lacerta ('lah-CERT-ah')
Genitive:Lacertae ('lah-CERT-ay')
Meaning:The Lizard
Short form:Lac
Area:201 square degrees
Co-ordinates:Right Ascension 22h 30', Declination +45°
Origin:Modern (17th Century)

Lacerta is a small northern constellation, invented by Hevelius in the 17th Century. The constellation is rather faint, with no stars brighter than 4th magnitude. The Milky Way 'flows' through part of the constellation, and there is one interesting object, BL Lacertae, although it is only visible in very large telescopes.

Lacerta is tucked in between Cygnus to the west and Andromeda to the east, and borders with Cassiopeia and Cepheus to the north and with Pegasus to the south. The constellation's most distinctive feature is a small 'W' of five stars, made from the stars beta, alpha, 4, 5 and 2 Lacertae.

History

More than half the constellations we use today were devised by the Ancient Greeks. Lacerta the Lizard is in an area of the sky fully visible from Ancient Greece, but the Greeks did not consider it interesting enough to merit a constellation. In the 16th - 19th Centuries, astronomers tried to fill in all the gaps betweeen the Greek constellations so that every star would be in a constellation. The Polish astronomer Johannes Hevelius (1611 - 1687) invented seven new constellations, including Lacerta.

Hevelius wasn't the only astronomer to consider this piece of sky. Augustin Royer, in 1679, had already invented the constellation Sceptrum et Manus Iustitiae, literally 'the sceptre and hand of justice' from the same stars as Lacerta plus some of the stars from Andromeda. Johann Bode, a century later, invented the constellation Frederici Honores (the Honours of Frederick) to commemorate Frederick the Great of Prussia, but the constellation was not popular. When the International Astronomical Union decided upon the official set of 88 constellations, they chose Lacerta as the constellation for this region of the sky.

Stars

None of the stars of Lacerta are noticeable enough to have acquired proper names, so they are known either by their Bayer designation (Greek letter plus constellation name) or Flamsteed designation (number + constellation name).

StarDesignationBrightness (m)Distance
(light years1)
Spectral classificationComment
α LacAlpha Lacertae+3.7780A1VDouble star
1 Lac1 Lacertae+4.131,000K3II-III 
5 Lac5 Lacertae+4.36500M0II+B8V 
β LacBeta Lacertae+4.43160G8.5IIIbCa1 
11 Lac11 Lacertae+4.46300K3III 

New General Catalogue (NGC)

The NGC catalogue is a list of interesting deep-space objects (that is, objects outside our solar system). It was compiled by Dreyer at Armagh Observatory based on the observations of Sir William Herschel. Lacerta has only three objects in the catalogue.

CatalogueTypeBrightness (m)Distance
(light years)
Remarks
NGC 7243Open Star Custer6.42,300Cluster mass is about 450 times that of the Sun.
NGC 7209Open Star Cluster6.73,300About 25 blue and white stars.
NGC 7296Open Star Cluster9.7  

BL Lacertae

The object known as BL Lacertae was discovered in 1929 by Cuno Hoffmeister (1892 - 1968). It was thought to be a variable star, so it was given a standard variable star designation, with two letters followed by the constellation name. Later it was found to be a rather unusual object which is not yet fully understood. It has given its name to a whole class of similar objects throughout the cosmos, which are called BL Lacertae objects, BL Lacs, or just Lacertids.

In 1968, John Schmitt was looking at the region using a radio telescope and discovered that BL Lacertae is a strong source of radio waves. Detailed examination later showed that it is not a star at all, but something much odder. There are four main findings:

  1. The light that comes from BL Lacertae does not have the 'thermal' spectrum of a hot body which normal stars and galaxies have. Instead it has a 'synchrotron' spectrum, with radiation right across the spectrum, in the radio, light, ultra violet and X-ray regions, with no noticeable emission or absorption lines. Such radiation is associated with electrons being accelerated to near the speed of light in a magnetic field.

  2. There are very faint traces of normal stars around BL Lacertae, although they are almost hidden by the glare of the synchrotron spectrum. Redshift studies of these stars show that BL Lacertae is very far away: 900 million light years. That's as far away as distant galaxies. For it to be visible at all from Earth, it must be pumping out vast quantities of energy, the equivalent of an entire galaxy.

  3. The variations in brightness which caused it to be originally categorised as a variable star happen very quickly, over the course of a few hours. This indicates that BL Lacertae is very small - changes in an object must start at one point and propagate across it at not greater than light speed, so if a change in brightness happens within a few hours, the object can be at most a few light-hours in diameter, which makes it smaller than our solar system. That may sound big, but it is a small region to be pumping out as much energy as an entire galaxy.

So what could be producing that sort of energy? The only known source of that sort of power is the destruction of matter as it falls into a black hole, so this is the current explanation offered by astrophysicists for all BL Lacertae objects. They imagine a giant spinning black hole, with a mass the equivalent of 100 million suns. Orbiting around it is a disc of dust and gas known as an accretion disc. Due to random collisions in the disc, there is a continuous stream of matter falling into the black hole.

For reasons not fully understood, there are two 'jets' shooting away from the black hole along the axis of rotation, carrying matter and high energy particles at speeds close to the speed of light. Such jets have been seen coming out of the axis of some distant galaxies. If a jet of a distant black hole were lined up in such a way that we were looking straight into the jet, then the energy would match that of a BL Lacertae object. Since this is the only known cause of something that bright, this is the preferred explanation. But it is not entirely satisfactory since the cause of the jets is not understood. There is currently much research being done on this topic by astronomers and astrophysicists.

Extrasolar Planets

Extrasolar planets are ones outside our solar system, orbiting other stars. They are a hot topic for astronomers, with new ones being discovered all the time. Because the distances involved are so huge, the planets have to be detected by indirect means, such as by analysing the effect their gravity has on their parent star.

One planet has been discovered so far within the Lacerta constellation. This star was discovered by the Hungarian Automated Telescope Project (HAT-P), so it has been given the name HAT-P-1 b (the letter b is standard for the first planet discovered in a star system). The project looks for stars which dim regularly as a planet goes in front of them. The information gathered gives the diameter of the planet as well as its orbital period. Doppler shift in the star's spectrum can be used to estimate the mass of the planet. This yields an interesting result: the planet is one of the least dense ever discovered, with a density less than half that of water.

Extrasolar Planets Table

Star name or
catalogue number
Planet
catalogue number
Planet mass
(Jovian scale)
Orbital period
(Earth days)
Year of discoveryComments
ADS 16402 BHAT-P-1 b0.5244.4652006Density = 0.378
(water = 1)

Because this planet orbits around its parent star in only four days, it must be very close to the star, so the surface temperature is likely to be far too hot for life as we know it.

Meteor Showers

The space debris which creates a meteor shower usually comes from the tail of a comet, as the Earth crosses the point where the comet has passed previously on its own orbit. Imagine a trail of breadcrumbs; now imagine breadcrumbs travelling at kilometre per second speeds and burning up in the Earth's atmosphere.

The meteor shower connected with this constellation is called the Alpha Lacertids, because they appear to radiate from a point in the sky close to the star alpha Lacertae. The meteor shower is visible in the period 3 - 12 July, but does not have an observed maximum. Hourly rates of meteors are typically less than eight.

1A light year is the distance light travels in one year, about 10 trillion kilometres.

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