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The discovery of the first eclipsing binary helped solve a long-standing puzzle in astronomy. The star Algol, in the constellation of Perseus, changes its brightness in an odd but regular way. Normally, Algol is a fairly bright star, but at intervals of 2 days, 20 hours, 49 minutes, it fades to one-third of its regular brightness. After a few hours, it brightens to normal again. This effect is easily seen, even without a telescope, if you know what to look for.

In 1783, a young English astronomer named John Goodricke (1764–1786) made a careful study of Algol (see the feature on John Goodricke for a discussion of his life and work). Even though Goodricke could neither hear nor speak, he made a number of major discoveries in the 21 years of his brief life. He suggested that Algol’s unusual brightness variations might be due to an invisible companion that regularly passes in front of the brighter star and blocks its light. Unfortunately, Goodricke had no way to test this idea, since it was not until about a century later that equipment became good enough to measure Algol’s spectrum.

In 1889, the German astronomer Hermann Vogel (1841–1907) demonstrated that, like Mizar, Algol is a spectroscopic binary. The spectral lines of Algol were not observed to be double because the fainter star of the pair gives off too-little light compared with the brighter star for its lines to be conspicuous in the composite spectrum. Nevertheless, the periodic shifting back and forth of the brighter star’s lines gave evidence that it was revolving about an unseen companion. (The lines of both components need not be visible for a star to be recognized as a spectroscopic binary.)

The discovery that Algol is a spectroscopic binary verified Goodricke’s hypothesis. The plane in which the stars revolve is turned nearly edgewise to our line of sight, and each star passes in front of the other during every revolution. (The eclipse of the fainter star in the Algol system is not very noticeable because the part of it that is covered contributes little to the total light of the system. This second eclipse can, however, be detected by careful measurements.)

Any binary star produces eclipses if viewed from the proper direction, near the plane of its orbit, so that one star passes in front of the other (see [link] ). But from our vantage point on Earth, only a few binary star systems are oriented in this way.

Astronomy and mythology: algol the demon star and perseus the hero

The name Algol comes from the Arabic Ras al Ghul , meaning “the demon’s head.” Fans of Batman comic books and movies will recognize that this name was given to an archvillain in the series. The word “ghoul” in English has the same derivation. As discussed in Observing the Sky: The Birth of Astronomy , many of the bright stars have Arabic names because during the long dark ages in medieval Europe, it was Arabic astronomers who preserved and expanded the Greek and Roman knowledge of the skies. The reference to the demon is part of the ancient Greek legend of the hero Perseus, who is commemorated by the constellation in which we find Algol and whose adventures involve many of the characters associated with the northern constellations.

Perseus was one of the many half-god heroes fathered by Zeus (Jupiter in the Roman version), the king of the gods in Greek mythology. Zeus had, to put it delicately, a roving eye and was always fathering somebody or other with a human maiden who caught his fancy. (Perseus derives from Per Zeus , meaning “fathered by Zeus.”) Set adrift with his mother by an (understandably) upset stepfather, Perseus grew up on an island in the Aegean Sea. The king there, taking an interest in Perseus’ mother, tried to get rid of the young man by assigning him an extremely difficult task.

In a moment of overarching pride, a beautiful young woman named Medusa had compared her golden hair to that of the goddess Athena (Minerva for the Romans). The Greek gods did not take kindly to being compared to mere mortals, and Athena turned Medusa into a gorgon: a hideous, evil creature with writhing snakes for hair and a face that turned anyone who looked at it into stone. Perseus was given the task of slaying this demon, which seemed like a pretty sure way to get him out of the way forever.

But because Perseus had a god for a father, some of the other gods gave him tools for the job, including Athena’s reflective shield and the winged sandals of Hermes (Mercury in the Roman story). By flying over her and looking only at her reflection, Perseus was able to cut off Medusa’s head without ever looking at her directly. Taking her head (which, conveniently, could still turn onlookers to stone even without being attached to her body) with him, Perseus continued on to other adventures.

He next came to a rocky seashore, where boasting had gotten another family into serious trouble with the gods. Queen Cassiopeia had dared to compare her own beauty to that of the Nereids, sea nymphs who were daughters of Poseidon (Neptune in Roman mythology), the god of the sea. Poseidon was so offended that he created a sea-monster named Cetus to devastate the kingdom. King Cepheus, Cassiopeia’s beleaguered husband, consulted the oracle, who told him that he must sacrifice his beautiful daughter Andromeda to the monster.

When Perseus came along and found Andromeda chained to a rock near the sea, awaiting her fate, he rescued her by turning the monster to stone. (Scholars of mythology actually trace the essence of this story back to far-older legends from ancient Mesopotamia, in which the god-hero Marduk vanquishes a monster named Tiamat. Symbolically, a hero like Perseus or Marduk is usually associated with the Sun, the monster with the power of night, and the beautiful maiden with the fragile beauty of dawn, which the Sun releases after its nightly struggle with darkness.)

Many of the characters in these Greek legends can be found as constellations in the sky, not necessarily resembling their namesakes but serving as reminders of the story. For example, vain Cassiopeia is sentenced to be very close to the celestial pole, rotating perpetually around the sky and hanging upside down every winter. The ancients imagined Andromeda still chained to her rock (it is much easier to see the chain of stars than to recognize the beautiful maiden in this star grouping). Perseus is next to her with the head of Medusa swinging from his belt. Algol represents this gorgon head and has long been associated with evil and bad fortune in such tales. Some commentators have speculated that the star’s change in brightness (which can be observed with the unaided eye) may have contributed to its unpleasant reputation, with the ancients regarding such a change as a sort of evil “wink.”

Questions & Answers

What kind of weather does Venus experience?
Michael Reply
why are the hyperlinks not working?
Gregory Reply
If the atmosphere blocks the view of Venus' surface, what am I seeing that looks like craters?
Michael Reply
clouds?
Michele
If the surface of Venus is shrouded by white clouds, making it impossible to see the surface, what looks like craters when view the planet?
Michael Reply
I don't Know! perhaps the mountains?
Michele
my question is if the surface of Venus is shining y not our earth
Tahir
when we look at venus we can't see any crater like things... check it once again
RIEM
and venus shines because of the clouds that are made up sulpher dioxide and sulphuric acid droplets. and the clouds are so dense. the case of earth is different
RIEM
also because of Venus's 70% albedo phenamenon
Shivam
and plus it reflects 70 percent of its light back into space earth don't shine because it's not a cloudy planet and its farther than the sun
americantuber
can I become an astronaut without taking mathematics as a subject in 11th class
UNIVERSAL Reply
If the surface of Venus is shrouded by white clouds, making it impossible to see the surface, what looks like craters? Are these atmospheric storms?
Michael
can a death of a massive star be the new big bang
Neeta Reply
No it might be a supernova. The big bang was much more massive.
Nick
It created the universe as we know it
Nick
the big bang is just a new beginning
Neeta
the death of a gigantic star makes it possible for a new begging
Neeta
Only in the local area of space
Nick
so many smaller universes are created and destroyed
Neeta
i think it created a supernova!
Michele
you still think? I am sure I created one supernova of my own
Neeta
I'm sure you did
Nick
neeta the death of a massive star could lead to an explosion that leads to a super nova and when the supernova explodes it becomes a nebula like messier 1
americantuber
a supernova remnant
americantuber
what is means by earthbound
Satyam Reply
you are earthbound, arnt we all earthbound? except the ISS etc.
Collins
I don't agree we may be earthbound because of our gross body/physical body...but there is also a subtle body which does not limit us to earthly existence
Neeta
good questions, as humans our habitat is earth, we are bound to this and have to alter ourselves to stay alive off land and outside the atmosphere, do you not agree? I'd guess you could also say the moon is earthbound. anything earthbound is restricted to earth. hope this helps, love your question
Collins
how do you describe astral projection
Neeta
Astral projection is spiritual. I'm assuming Satyam was talking about the physical world.
TheDirtyGhost
ah yes spiritually we are bound only to what we chose, like nothing, I like to think that way
Collins
anyone here also participating in seti@home?
dreamer
what is seti?
Mahesh
what has extraterrestrial entity got to do with this why @seti project..we are discussing human existence on earth and beyond earth
Neeta
please join me at the cosmic Diner where all your intergalactic dreams come true.
Alba
I was just thinking what if somebody ignorantly mistaken a cosmic microwave Background with a conventional microwave we be in a lot of trouble.
Alba
don't kno
dreamer
You mean the Restaurant at the end of the Universe...right we can come back and visit again but not sure of the mode of payment
Neeta
yeah I'm already heading there
dreamer
how to read stars
Amresh Reply
you don't
Max
astrology i guess
Uttam
astrologer is the study of star systems and constellations...but astronomy is the study of formation of universes multiverses birth and evolu of stars
Neeta
cool
Uttam
what is astronomy
babul Reply
study of formation evolution and the death of star star systems and galaxies
Neeta
astronomy is the study of universe
UNIVERSAL
astronomy and physics are basically the same except physics is the study of the motion and behavior of the universe
americantuber
What is 12 constellation of zodiac and why it is important to study astrology
MUHIBULLAH
And also what is main purpose of these 12 constellation of zodiac in astronomy
MUHIBULLAH
astronomy and astrology with two different things
Deja
it Is the study of universe and of life speaking generally! i think!
Michele
universe is born 13,7 miliardi of years with big Bang if i remember well! but there are also other theories for the universe, speaking generally! i remember so!
Michele
universe is actually about 13.7 billion years old
Nick
is astrology like astronomy
debjani Reply
Astrology is a lower dimension.. astronomy is much more vast and multidimensional
Neeta
yes.. astrology is about constellations only, and astronomy is about all stars, galaxies, gravity, dark matter, dark energy.. etc everything including astrology
Mahesh
can you become an astronaut without taking mathematics as a subject in 11th class
UNIVERSAL
I was reading the chapter on Cosmic Microwave Background. And, I can not seem to find it now. If anyone could help me find that portion of the Astronomy Textbook I would really appreciate it. AZ
Alba Reply
Where or which chapter discusses Cosmic Microwave Background?
Alba
29.4
Andrew
29 the big bang ch 29.4
Andrew
yes
pratham
interesting
Orlando
what's cold dark matter?
pratham Reply
Dark matter even the science community is not sure what it is...!!! That's why they have some of the smartest math wizards around to try to figure out that puzzle!
Gregory
How are Roche Worlds formed?
AlteredEdge Reply
In need to read some books about Astronomy so how can in get it actually leg live in Ethiopia can uh help me with that?
Mom Reply
I am sri Sharan .m .what is my best favourable numbers
Madhesh Reply
how can we know it
sruthi
😂😂
Rango
bahut hard
Rango
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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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