2.1 - star magnitudes

how did hipparchus classify stars?

by their brightness to the naked eye (i.e., intensity of light received);

• 1 = brightest

• 6 = dimmest

what is brightness?

intensity of light received

which feature of stars did hipparchus classify his stars by?

their brightness to the naked eye

in hipparchus’ star classification, which objective point was the brightness measured from?

the naked eye

how did hipparchus’ scale range?

1 = brightest

6 = dimmest

for hipparchus star classification, what does a star of brightness 1 mean?

it was the brightest

for hipparchus star classification, what does a star of brightness 6 mean?

it was the dimmest

what were the problems with hipparchus’ classification?

• he did not measure the brightness of the sun, so questions were raised where to class the sun

• only considered stars emitting visible light

• subjective, therefore not accurate

• the invention of telescopes led to it’s discontinuation

why was the invention of telescopes a problem for hipparchus’ classification?

was hipparchus’ scale objective or subjective?

subjective

why was hipparchus’ scale subjective?

• it relied on the observer’s own opinion on how bright a star is, which could vary between observer’s

• it didn’t account for any problems the observer had with their vision that could affect how they perceived the brightness of a star

why does the subjectivity of hipparchus’ scale affect the classification’s accuracy?

the brightness of stars could not be considered as the objective truth because it’s classification is purely subjective, which means the value of brightness could vary between different observers

what didn’t hipparchus consider in his scale?

the brightness of our sun

why was it a problem that hipparchus didn’t measure the brightness of our sun?

because now we don’t know where to class the sun on his scale

what were the only kinds of stars hipparchus considered in his scale?

stars that emit visible light

why did hipparchus only consider stars that emit visible light in his scale?

his scale considered brightness. if a star did not emit visible light, it’s light intensity could not be measured, therefore stars that didn’t emit visible light would not be considered

why was it a problem that hipparchus only considered stars that emit visible light in his scale?

it did not consider a wholistic view of the stars in the universe, as not all stars emit visible light

what is a light year?

• the distance light travels through space in 1 year

• 9.5 × 10^15 m

how long does it take light from the sun to reach earth?

500s

how long does it take light from the sun to reach jupiter?

40 mins

how long does it take light from the sun to reach pluto?

6 hours

derive a light year

• 1 light year = speed of light x time in seconds for one year

• 9.45 × 10^15 m =9.45×10¹⁵ m x 3 × 10^8 × 31.56 million seconds

what is the unit of measurement for a light year?

metres

how long does it take light to travel across the milky way galaxy?

100,000 years

what is a spiral arm?

characteristic feature of a galaxy - spiral shaped regions of enhanced brightness from stars and planets

where is the sun and nearby stars located in the milky way galaxy?

in the spiral arms

how many stars are in a galaxy?

almost a million million (trillion)

what are galaxies?

assemblies of stars prevented form moving away from each other by their gravitational attraction

why can’t stars in a galaxy move away from each other?

because of their gravitational attraction

are stars in a galaxy prevented or free to move away from each other?

prevented, because of their gravitational attraction

how far away are galaxies from each other?

millions of light years apart

what makes up the space between different galaxies?

empty space

how can astronomers tell if a star is relatively near to earth?

their position shifts against the background of more distant stars as the earth moves around its orbit, known as parallax

HERE

what is parallax?

a way to determine whether is star is relatively close to earth, as the position of nearby stars shift against a background of distant stars as the earth moves around it’s orbit

why does parallax occur?

the line of sight to a nearby star changes direction over 6 months, as you’re viewing it from a diametrically opposite position when the earth is halfway through its orbit

in which time intervals can we employ parallax to tell the determine nearby star distance from earth?

6 months, i.e., halfway through the earth’s orbit around the sun

why do we employ parallax every 6 months to determine the distance of a nearby star?

a year is 12 months, so 6 months is halfway through the earth orbit. this means the line of sight of sight to a nearby star is in a diametrically opposite direction compared to 6 months before