2What is a star? a ball of gas held together by its own gravity gravitational force continually tries to collapse the star by pulling gas molecules toward its centerGravity is counteracted by the pressure of the hot gas inside the center of the starGravity pulls inward, and the pressure pushes outwardThe two forces balance each other and the star maintains a spherical shape
3The Sun The Sun is our own special star Fueled by nuclear fusion reactionsan active star that displayssunspotssolar flareserupting prominencescoronal mass ejectionsThese phenomena impact our near-Earth space environmentdetermine our "space weather."
4Magnitude Invented by the ancient Greeks around 150 B.C. 2 kinds of magnitudeApparent - how bright stars look to us in the sky from here on EarthAbsolute – how bright a star would be if all the stars were equal distance away from the Earth
5Classification Stars are classified by temperature and color O stars are the hottestExtremely bright, very rareM stars are the coolestVery common, fairly dimColor of star is connected to temperatureHot stars are blueCool stars are redIn between stars are white, yellow, or orangeSun has a medium temperatureYellow star
7StargazingHow we interpret our observations depends on our point of viewMedia gives us many points of viewWe have a better understanding of the natural worldWe don’t rely on legends to explain the movements of the sun, moon, and stars.Stars hold the mystery of blazing nuclear furnaces scattered in the vastness of space.
8Historical Observations Early Greeks gave names to star-shaped figures from their religious and cultural stories.These stories are called myths.For centuries man has wondered whether the numberless specks in the sky could affect their lives.People who studied the stars were called astrologersAstrology became a religious practice.Became part of the the Babylonian religionThe zodiac was a sacred pathway for the sun and the planetsStars also were signposts for sailors and travelers.
9Historical Observations Ancient people told time by the skyMoon measured the monthStars marked the year and seasonObservers noticed that the stars set 4 minutes earlyier every night.As people studied the sky, they observed at least four different motions:Most stars rise in the east and set in the west;Both the sun and moon rise in the east and set in the west;The sun appears over the horizon farther north in summer and farther south in winter;The moon does not always rise at the same time.
10Constant MotionThe Earth turns on its axis at 1,600 kilometers per hourThe Earth moves around the Sun at 107,200 kilometers per hourOur solar system moves around the Milky Way at 69,200 kilometers per hour.The Milky Way Galaxy is moving toward the star Vega at 20 kilometers per SECONDOur solar system is located in the Milky Way Galaxywe cannot look at the Milky Way from the outside to observe its size and shapeOuter space is so large that a measurement was invented to express the large distances.A light year is the distance that light travels in one year.One light year is almost 6 trillion (6,000,000,000,000) miles
11Constant MotionInstruments measure the speed of light as 186,000 miles per secondThe nearest star, besides the Sun, is Proxima CentauriIt is 4.3 light years away
12Star ChartsConstellations are star groups that seem to travel together in space.Star charts are used to help follow the starsEach season hasits own pattern ofstarsNorthern Circumpolar Constellations
13Winter Sky February is the best month to view stars Orion is the best known constellation3 stars make up Orion’s beltBetelgeuse marks his right shoulderBetelgeuse has a diameter of 400 million milesRigel is Orion’s left kneeTo the southeast is Sirius, the dog starIt is the nose of Canis MajorTo the northwest is Taurus the BullThe horns contain AldebranPleaides, the seven sisters, are located in the shoulder of the bull
15Spring Sky Best known constellation – Ursa Major, the big bear 2 end stars point to Polaris, the North StarNorth StarNot one of the brightest starsCan be used to find directionIs the end star in the handle of the Little DipperSoutheast of the arc in handle of Big Dipper leads to Arcturus.Arcturus is in BoötesEast horizon is LibraMost prominent constellation is LeoLooks like a question markStar Regulus is the period
17Summer Sky The summer triangle Lyra, the harp – contains Vega East of Lyra is Cynus, the swan – The northern crossBrightest star is Deneb in the tail of the swanBelow Cynus to the southeast is Aquila, the eagleBrightest star is AltairAltair, Vega, Deneb make up the summer triangleWest of Lyra is HerculesHercules’ foot is on the head of Draco, the dragonSouth of Lyra is Sagittarius, the archerOn southwestern horizon is Scorpius, the scorpion
21The Perennial Sky Polar stars are found around Polaris These stars never set in the northern hemisphereThey are called circumpolar starsInclude constellations:Little Dipper, Big Dipper, Draco, Cassiopeia, Cepheus, Camelopardalis
22What is a constellation? a group of stars that seem to form a picture in the night sky.The Greeks recognized and named forty-eight constellations. Many of these constellations were also recognized by the Arabs, Egyptians, and the Babylonian. In 1928, the International Astronomy Union fixed the boundaries of the eighty-eight constellations in the sky. The names of the constellations are given in Latin, because Latin was once the language of learning.
23What is a constellation? For convenience, the ancients named the constellations after a certain figure that it seemed to form in the sky. They refer to mythological or legendary men, women, and animals.For thousands of years, people have used their knowledge of the constellations to guide them from place to place.By knowing the positions of the constellations, it is possible to locate stars, planets, comets, and meteors.The constellations appear to move westward as the Earth rotates around the axis. For this reason, certain constellations can be seen only during one of the seasons of the year.
24AndromedaAndromeda's mother, Queen Cassiopeia, bragged that she was prettier than the sea nymphs. The nymphs complained to Poseidon, who in turn sent a monster to destroy her land. The queen and her husband, King Cepheus, were told to sacrifice their daughter to save the country.Andromeda was chained to a cliff for the monster, called Cetus. Just as the monster was ready to bite down on the maiden, Perseus rescued her. Perseus and Andromeda were put in the sky along with Cepheus, Cassiopeia and Cetus.
25AquariusIn Greek myth, Aquarius was Ganymede, the young boy kidnapped by Zeus. Zeus sent his eagle, Aquila, to snatch Ganymede out of the fields where the boy was watching over his sheep.Ganymede would become the cupbearer for the Olympian gods.
26AriesAries represents the ram of the Golden Fleece. Zeus, a god, once married a mortal woman named Nephele. They had two children together. Their names were Phrixus and Helle. Once as a gift, Zeus gave Nephele a winged-ram of Golden Fleece. Soon Zeus married another mortal woman, her name was Ino. She hated Nephele’s children and wanted to kill them.
27AriesTo keep her children safe, Nephele sent them away on the back of the winged-ram. The ram flew east. One day as the ram and the children were gliding through the air Helle fell off. She fell right between Europe and Asia , but Phrixus made it safely to Colchis . There, Phrixus sacrificed the ram and gave its Golden Fleece to King Aeetes. Later, Zeus decided to put the figure of the ram in the sky as a constellation, so he would never forget Nephele, Helle, or Phrixus
28BoötesBoötesgenerally considered to be a Herdsman, as he eternally shepherds the stars around the North Pole.first recorded appearance being in Homer's Odysseysqueezed between Canes Venatici and Hercules, with Virgo to the southThe northern border touches both Ursa Major and Draco
29BoötesThe association of the Dipper with the Greater Bear leads Boötes to be identified with the story of Zeus' seduction of the maiden Callisto, who was turned into a bear as result of the jealousy of Zeus' spouse, the goddess Hera. Callisto was the daughter of King Lycaon of Arcadia. She bore the boy Arcas as the result of her liaison with Zeus.
30CancerAccording to an ancient Greek legend, the figure of a gigantic crab was placed in the nighttime sky by the goddess Hera to form the constellation Cancer. Hera swore to kill Heracles, the most famous Greek hero. Hera attempted to kill Heraclesin many different ways, but each time his incredible physical strength allowed him to survive. The Romans called him Hercules. Hera cast a spell of madness on Heracles, causing him to commit a great crime. In order to be forgiven, he had to perform twelve difficult tasks. One of these tasks was destroying the terrible nine-headed water-serpent, Hydra.
31CancerDuring the battle between Heracles and Hydra, the goddess Hera sent a giant crab to aid the serpent. But Heracles, being so strong, killed the crab by smashing its shell with his foot. As a reward for its service, Hera placed the crab's image in the night sky.
32Canis MajorA magical dog which was destined never to surrender a chase. It was first bestowed on Europa by Zeus, who passed it to her son Minos, and from him to Procris and Cephalus. The last of these set it to hunt down the Teumessian fox, which was destined never to be caught. To resolve the contrary fates of the two animals,Zeus placed them amongst the stars as the constellations Canis Major and Minor to play out the chase unresolved for eternity.
33Canes VenaticiIt represents the two dogs Asterion and Chara, both held on a leash by Boötes as they apparently chase the Great Bear around the North Pole.With one exception, the constellation's stars are quite faint, fourth- and fifth-magnitude stars
34CapricornusThis constellation depicts the result of the sudden appearance of the earthborn giant Typhoeus. Bacchus was feasting on the banks of the Nile at the time, and jumped into the river. The part of him that was below water was transformed into a fish, while his upper body became that of a goat. From this point of view, he sawthat Typhoeus was attempting to tear Jupiter into pieces; he blew a shrill note on his pipes, and Typhoeus fled. Jupiter then placed the new shape of Bacchus in the heavens out of thanks for the rescue.
35CassiopeiaA Queen of Ethiopia, mother of the lovely Andromeda. When she boasted that her daughter was more beautiful than the Nereids, Poseidon sent a sea monster to devour the girl. She was rescued by Perseus, who slew the beast. As a memorial the whole family were placed amongst the stars, but Cassiopea because of her pride, was set to hang eternally upside down on her throne.
36CepheusA King of Aethiopia and father of the lovely Andromeda. He was forced to sacrifice his daughter to a sea monster because the boasts of his wife Cassiopeia offended the gods. But the hero Perseus slew the beast and rescued her. As a memorial the whole family - Cepheus, Cassiopeia, Andromeda and Perseus - were placed amongst the stars.
37Coma BerenicesComa Berenices is sandwiched between the Hunting Dogs, Canes Venatici to the north, Virgo to the south, Leo on the west border and Boötes on the east border.When the King went to war, Berenice promised her hair to Aphrodite, the Greek Goddess of Love, if her husband returned safely and victorious. After the king’s return Berenice kept her promise though the loss was hard. Aphrodite herself brought the hair to the stars.
38CygnusWhen Zeus wanted to seduce the goddess Nemesis he transformed himself into a swan, and bade Aphrodite in the guise of an eagle pursue him into her lap. As a memorial of this successful ruse he placed an eagle and swan in the sky as the constellations Cygnus and Aquila.
39Draco circumpolar constellation visible all night from northern latitudeswinds around the little dipperAt one time Draco was quite a bit larger when the ancient Mesopotamians gave the dragon large wings which wound around Ursa Major.The Greek philosopher Thales lopped off the wings in the sixth century BC.
40DracoDraco the dragon is famous throughout mythology. One of the more popular stories involves Heracles and the twelve labors. Gaia gave Hera a golden apple tree when she married Zeus. Hera put the tree in the garden to be guarded by the Hesperides and a dragon called Ladon.Heracles asks Atlas to gather the apples while he and Athena held up the sky. Atlas, thinking he could trick Heracles into holding the sky forever, gladly accepted the mission. When he returned with the apples, Heracles slipped the sky back on Atlas' shoulders.
41GeminiThe constellation represents the twins Castor and Pollux. The Twins are really only half-brothers. They share the same mother Leda, but have different fathers. Castor's father, and Leda’s husband, was Tyndareus, the king of Sparta. The father of Pollux was none other than Zeus. Zeus visited Leda on her wedding night disguised as a swan. Castor was mortal and Pollux was not. When Castor was killed in battle, Pollux asked if he could share his immortality with his brother. His wish was granted and they were placed in the sky together.
42HerculesHercules is the figure of the most famous Greek hero, Heracles. Heracles was a muscular man endowed with extraordinary courage and physical strength. The ancient Romans called him Hercules. Heracles was the son ofthe sky god Zeus and the queen of Tiryns, Alcmene. When Hera discovered Zeus' infidelity, she decided to kill the child born from the Zeus' relationship with Alcmene. That child was Heracles. Hera placed two serpents in Heracles' cradle, but the child strangled the snakes with his bare hands, clearly
43Herculesrevealing his divine nature. This and many other heroics made Heracles famous throughout the ancient world. Hera was so annoyed at Heracles' growing fame that she cast a spell of madness over him. Out of control, Heracles killed his own wife and children.His remorse was so profound that when he returned to his senses he could find no peace of mind. He visited the oracle of Delphi to see how he could demonstrate his remorse.The oracle advised him to obey the orders of Eurystheus, the king of Tiryns and Mycenae. Eurystheus ordered Heracles to
44Herculesaccomplish twelve difficult tasks (the so-called Twelve Labors of Heracles). Heracles completed the twelve labors and is celebrated to this day for his great courage and strength.
45HydraHydra lived in a swamp in a land called Lerna. The second labor of Heracles was to kill the monster. It was no easy task because the Hydra had 9 heads, one of which was immortal. During the battle each time Heracles cut off a head two more heads sprouted in its place. Heracles was losing the battle until he took a torch and burned each neck as he cut off the head. That prevented any new heads from sprouting. Finally the Hydra only had its single immortal head remaining. Heracles cut it off but because that head could not be killed he got rid of it by burying it under a massive rock. And that was the end of the Hydra.
46LepusLepus the hare has a number of origins. According to one story, Orion the famous hunter (and the constellation right above Lepus) loved to hunt hares, and so Lepus was placed in the sky for Orion's benefit. In another story, Lepus represents the hare so often associated with the moon. While we tend to see a man in the moon, many other cultures have seen a hare, and have many stories to tell about it. The Arabs believed that the four brightest stars in Lepus represented four camels drinking from the river Eridanus, another nearby constellation. The early Egyptians believed Lepus to be the boat of Osiris.
47LepusPerseus had to fetch the head of the Gorgon Medusa whose head could turn anyone who looked at it into stone. With the help of Hermes, Perseus succeeded. On his voyage back to Seriphos, he met the beautiful Andromeda in Ethiopia and married her. When his triumph became known, he was invited to the city of Larissa to participate in funeral games in honor of the king.47
48LeoThe first on the list of Hercules' labors was the task of killing the Nemean Lion. A giant beast that roamed the hills of the Peloponnesian villages causing havoc. Hercules' arrows bounced harmlessly off of the lion's body, his sword bent in two and his wooden club smashed to pieces. Hercules had to wrestle the beast finally choking it to death.Hercules then wrapped the pelt of the lion around his body to protect himself from his second labor, killing the poisonous sea serpent Hydra. The lion found its way to the heavens to commemorate the great battle with Hercules.
49LibraLibra is a zodiacal constellation. It represents the balance or scales, and is one of the oldest constellations.Although now associated with Virgo, a goddess of justice who had scales as the emblem of her office, it was once associated with the fall equinox. On that day, the days and nights are of equal length (i.e. the moon and the sun are in balance).Libra is represented in the heavens next to the hand of Virgo.
50OrionIn Greek mythology, Orion was a famed Boeotian hunter. In one account he was the son of Poseidon. Orion was blinded by Oenopion, whose daughter he had raped, but his vision was restored by the rising sun. Stories concerning Orion's death vary. The most common relates that as he attempted to assault Artemis, or one of her nymphs, he was stung by a scorpion sent by Apollo or by Artemis herself. Both the scorpion and Orion later became constellations.
51PegasusIn Greek mythology, when Perseus slew the Medusa drops of the monster's blood fell into the sea. Poseidon, god of the sea, commanded that the mixture of blood and sea-foam give rise to Pegasus, the Winged Horse. Now this wonderful horse was coveted by many a man but only one, the heroBellerophon, was able to tame him. Bellerophon knew that it would be difficult to capture Pegasus so he asked Athena, the goddess of wisdom, for help. Athena listened to Bellerophon and gave him a golden bridle whose magic could tame the Winged Horse. Bellerophon knew Pegasus'
52Pegasusfavorite watering hole and he waited there in hiding. When Pegasus came to get a drink Bellerophon held out the golden bridle and Pegasus willingly came to his new master. At first Bellerophon found it difficult to control the flying horse but in a short time the two soared as if they were made for each other.
53PegasusDuring his travels Bellerophon came to the country of Lycia and found the land in complete disarray. A monster had been terrorizing the countryside and the king of Lycia asked Bellerophon for help. The monster was the Chimaera, part goat, part lion, and part dragon. No one had been able to kill it and few even had the courage to try. Bellerophon accepted the king's request. He flew on Pegasus to the Chimaera's lair and after a terrific battle slew the monster and saved the kingdom. Pegasus and Bellerophon were to have many grand adventures together but they were not destined to remain companions forever. After a while Bellerophon became very prideful and thought that he deserved a place with the gods. When he flew Pegasus towards heaven Zeus became furious with his arrogance. Zeus sent a gadfly to bite Pegasus. He bucked so strongly that Bellerophon fell off and plummeted to earth. Bellerophon survived the fall and wandered about the countryside lonely and lame until the end of his days. Pegasus continued flying up to heaven and worked with Zeus by carrying his thunderbolts.
54PerseusPerseus was an ancient Greek hero. His mother was Danae, the daughter of Acrisius, the king of Argos. When a prophecy revealed to Acrisius that his grandson would kill him, Acrisius imprisoned his daughter Danae to keep her chaste. Zeus, however, fooled Acrisius' precautions by entering the prison disguised as a shower of gold. When Acrisius discovered that Danae had given birth to Perseus, he had the mother and the son thrown into the sea in a chest of wood. Luckily they reached the island of Seriphos where the king Polydectes offered them hospitality and protection. Perseus was secretly raised on the island and became a courageous young man. He was sent to complete a dangerous mission.
55PerseusDuring the games, Perseus threw a discus and accidentally hit his grandfather Acrisius, who was watching the games, unaware of his grandson's presence. Acrisius died fulfilling the prophecy.
56Pisces One day Typhoeus, an earthborn giant, appeared and scared all the gods, forcing them to take different forms to flee. Jupiter transformed into a ram, Mercury into an ibis, Diana into a cat, and Bacchus into a goat and they all fled. Cupid and his mother Venus, who were bathing in the banks of the Euphrates River at the time, transformed themselves into two fish. These two fish were later known as Pisces the fish. Later, to immortalize this event Minerva placed the figures of these two fish in the sky as a constellation.
57SagittariusThe zodiacal constellation Sagittarius represents the centaur Chiron. Most of the centaurs were regarded in myth as bestial--they were, after all, half horse. However, the ancient Greeks had a great deal of respect for the horse, and so were reluctant to make the centaurs entirely bad. In fact, Chiron was renowned for his gentleness. He was an excellent archer, musician, and physician, and tutored the likes of Achilles, Jason, and Hercules.Chiron, however, was accidentally shot and wounded by Hercules. The arrow, which had been dipped in the poison of the Lernaean Hydra, inflicted great suffering on Chiron--so great, in fact, that even the talented physician could not cure himself. In agony, but as an immortal unable to find release in death, Chiron instead offered himself as a substitute for Prometheus.
58SagittariusThe gods had punished Prometheus for giving fire to man by chaining him to a rock. Each day an eagle would devour his liver, and each night it would grow back. Jupiter, however, had at the request of Hercules agreed to release Prometheus if a suitable substitute could be found. Chiron gave up his immortality and went to Tartarus in place of Prometheus; in recognition of his goodness, Jupiter placed him in the stars
59ScorpiusTo the ancient Greeks, the constellation Scorpius was the image of a scorpion. The constellation was related to the death of the giant hunter Orion by an enormous scorpion. There are several different stories about Orion's death. According to one variant, Orion once met the goddess of the chase, Artemis while he was sailing to the island of Crete. Artemis loved hunting as well as Orion. They began hunting together and enjoying each other's company. Orion became so infatuated with Artemis that he tried to rape her. The goddess avenged herself and set a scorpion on him. No matter how hard he tried, Orion could not defeat the scorpion.When Orion tried to escape, the scorpion stung him to death with its poisonous tail. As a reward for its services, Artemis placed the image of the scorpion among the stars. To this day, the constellation Scorpius appears in the sky to be ever pursuing the constellation Orion.
60TaurusZeus, a god, fell madly in love with a mortal woman named Europa. He wanted to meet her, but he didn’t want her to know his true identity, just yet. He went to her in the form of a snowy white bull. At first she was scared, but when she saw the bull meant no harm she made a wreath of flowers and put it around his neck. Then they started to play with each other. Soon Europa became very comfortable with the bull. She climbed onto the bull’s back and he swam out to sea with her.
61Taurus He took her to Crete and revealed his true identity to her. They had three children together. Eventually Europa died. Zeus never wanted to forget her so he put the figure of a bull in the sky as a constellation to always remember he was in the form of a bull when he first met her.
62Ursa Major.Callisto was a river goddess. Callisto was the favorite companion of the moon goddess Diana. She used to accompany Diana on the hunt. One day the god Jupiter saw the beautiful Callisto and fell in love with her. They had a boy named Arcas. Jupiter's wife Juno was mad and changed Callisto into a bear.One day, Arcas shot an arrow at Callisto because he thought she was a bear. At the last moment, just before the arrow struck, Jupiter tossed Callisto and her son Arcas into the heavens as the star formations Ursa Major and Boötes. Ursa Major is known as the Great Bear, with Boötes, the Bear Warden. Arcas is always standing next to her.contains the Big Dipper. The handle of the Dipper is the Great Bear's tail and the Dipper's cup is the Bear's flank
63Ursa MinorIn Greek myth, Zeus was having an affair with the lovely Callisto. When his wife, Hera, found out she changed Callisto into a bear. Zeus put the bear in the sky along with the Little Bear, which is Callisto's son, Arcas.contains the Little Dipper. The handle of the Dipper is the Little Bear's tail and the Dipper's cup is the Bear's flank.
64VirgoNamed for the Greek goddess Demeter, the Earth-goddess, and is associated with the arrival of spring and bringer of the growing season. Hades, the God of the Underground, fell in love with Demeter's daughter, Persephone. He promised himself that he would marry Persephone, making her his queen. Demeter would not stand for this. One day Hades, in a black chariot drawn by four great black horses in golden harness and reins, rode up to Persephone and carried the girl off with him back to the Underworld. Demeter became worried when her daughter did not return home. She sent out to search for her daughter. Demeter searched with out food or sleep. When Zeus pleaded with her to return to Olympus and accept Persephone's marriage to Hades, she refused and continued her search.
65Virgo People everywhere were starving and could not understand how their Earth-goddess could be so cruel. Zeus sent Hermes, Messenger of the gods, to visit Hades and tell him that Persephone must return with him. Hades, knowing that the gods of heaven were stronger than he, agreed to let Persephone go. There was a happy reunion, but Persephone told her mother that she loved her husband Hades. Zeus solved the problem by saying that Persephone would spend half of her time in the Underworld with Hades and the other half on Olympus with her mother. In this way, winter comes when Persephone goes down to the Underworld to be with Hades. When Persephone returns to Olympus, the winter cloak of death melts and there is a rebirth of life over the land and the crops begin to grow.
66Geocentric TheoryThe idea that the Earth was the center of the universeAristotle’s viewPtolemy’s view
67AristotleGreek philosopher Aristotle supreme authority in scientific matters for almost 2000 years after his deathGeocentric theory – earth is the center of the universeThe planets are fastened to a series of crystalline spheres concentric with the earth.Contradicted his observations
68Ptolemy Modified Aristotle’s model Also thought that the Earth was fixed at the center of the heavensSun, moon and planets revolved around EarthExplained that each planet moves in a small circleThe center of this small circle is carried on the circumference of a larger circleThe center of the larger circle is Earth
69MeteorsA bright tail or streak of light that appears in the night sky when a meteoroid enters the Earth’s atmosphere. The friction with the air causes the rock to glow with heat.A piece that manages to get through the atmosphere and hits the ground is known as a meteoriteAvg speed of a meteoroid is 85,000 mph2 tons of meteoric dust falls on the Earth every 24 hours
70Meteors Most dust is made of iron & nickel Meteor showers occur when a large number of meteoroids appear together and seem to come from the same area in the sky.They are named according to the constellations from which they appear to fall.
71Red Shifta certain kind of change observed in the spectrum of most stars and galaxiesThe wavelengths of light in the spectra of many types of nearby stars are well known.When spectra are observed of the same types of stars lying at a great distance, however, the wavelengths usually appear longer.That is, they are shifted toward the red (longer-wavelength) end of the spectrum
72Hubble Constant Developed by Edwin Hubble in 1920s Discovered that galaxies were heading AWAY from the Earth in all directionsShowed that more distant galaxies were moving away from Earth faster than closer galaxiesHe found that the red shift was greater for distant galaxies
73Hubble ConstantHubble Constant is the relationship between distance and velocity.Expressed in kilometers per second per megaparsecThese six galaxy clusters are a subset of the 38 that scientists observed with Chandra, with distances ranging from 1.4 to 9.3 billion light years from Earth, to help determine the Hubble constant.
74Cepheid VariablesCepheid variables can help us to determine distances to celestial objects.Cepheids are referred to as "cosmic mileposts" or "standard candles" of the universe because they help us measure interstellar distances.Cepheids get bright rapidly...then dim gradually over periods of time ranging from a few days to a few months.The period of time a Cepheid takes to complete one pulsation is directly related to the star's intrinsic brightness.Its intrinsic brightness helps us find the distance to the star. If we know how bright the star really is and we can see how bright the star appears to be, then we can measure its distance.If a Cepheid appears brighter than it should, it's closer to us.If it appears dimmer than it should, it's farther away.