Why do Planets appear brighter than Stars
“How brave the moon shines in her skin, outnumbered by the stars.”
This beautiful line by Angie Weiland has always made me wonder, making me question myself
whenever I look up at the night sky.The instance I look at the sky, I observe a lot of stars,
some barely visible to our naked eyes, others comparatively brighter than the other.
Our moon, in its full brightness, lightens the sky enough for me to read a book.Why is
it that some objects are bright, while others are dim?
Why does the moon shine brighter than other objects in the sky?
Is that object a planet, Is it a star, or is it a satellite?
Why do planets appear brighter than stars?
What is the major difference between a planet and a star, and what factors affect the brightness
of a star and a planet?
Today, we’re going to share our thoughts on this puzzling topic with you, so that the
next time you look up in the sky, you have the knowledge to differentiate between a star
and planet.
You wouldn’t be shocked to know that Venus is the third brightest object in the night
sky, only next to the moon and the ISS.
Before we move on to further topics, let's discuss a bit more, about the difference between
a planet and a star.
A star is a large,luminous object composed of gasses like hydrogen and helium.
Stars emit their own light through complex processes known as nuclear fusion, during
which they convert hydrogen to helium, emitting huge amounts of heat and light,lighting up
entire space in proximity to them.
Planets, on the other hand, are massive rocky objects with a gaseous atmosphere.
Planets are formed with the star’s residue matter in space.
They are generally found in orbit around stars and don’t have their own light.
They can reflect the light from their parent stars or other sources of light from the cosmos,
yet they still shine brighter than other stars whose light reaches us.
The simplest example of this is our own solar system.
But, the question still remains unanswered.
Why does Venus shine brighter than most of the brightest stars known to mankind?
Why do Planets appear brighter than Stars?
Most importantly, how do we define the brightness of cosmological objects and what is the unit
to measure them?
What factors affect the brightness of cosmological objects like stars and planets?
Distance of celestial objects from our reference frame plays the most important role in deciding
the brightness of objects.
It is due to this reason that faraway stars are really dim as compared to planets.
Planets, such as venus or jupiter are relatively very close to our reference frame.
Hence, they appear brighter than the brightest stars in the night sky.
However, if distance was the only reason, we wouldn’t be doing a video on this topic.
There are many different factors deciding the brightness of stars and planets separately.
To grasp the secrets behind all our questions, one needs an in-depth knowledge of advanced
cosmological terms such as brightness, luminosity,Intensity and many more.
In this video, we will try to keep your understanding to the best, and complexity to the minimum
requirements.
Let's first discuss the factors which decide the brightness of stars, as the factors deciding
brightness of stars do not necessarily decide the brightness of planets.
Magnitude of the brightness of stars can be measured in two ways.
The first method of measurement is apparent magnitude and the second being absolute brightness.
Apparent brightness is measured using Photometry.
When the brightness of an object is measured by an observer as seen by on the earth, while
the celestial object being observed is at its greatest distance from the Earth, it is
known as the apparent brightness of the object.Apparent brightness is measured as negative of logarithmic
value of ratio of brightness of the celestial object to that of the sun.
As we take the measurements in negative, it is clear that more the value of apparent magnitude
of brightness of the star, the fainter the star is and the less the value, the brighter
the object appears.
For example, the value of apparent magnitude of venus is approximately -6 when it is brightest
and -4 when it is at maximum distance from earth.
On the other hand, the apparent magnitude of the brightest star,Sirius, is about -1.
Average apparent magnitude of night stars is nearly +5 to +4.
Absolute magnitude of brightness is the apparent magnitude of the celestial object when it
is at a distance of 10 parsec, or 32.60 light years.Calculation in absolute magnitude is
much more accurate than apparent brightness.
Magnitude of brightness can be calculated for all objects including the sun, other stars
as well as the moon.
Apparent brightness depends upon distance of body from the reference frame.Hence,Calculating
apparent magnitude of celestial objects provides an approximate comparison of their brightness
as seen from the earth, but cannot provide comparison of brightness of various celestial
objects,unlike absolute magnitude.The apparent brightness of a star depends on both its luminosity
and its distance from Earth.
So, it would be best to talk about absolute magnitude while comparing the brightness of
various objects.
Another factor affecting brightness of celestial objects is the amount of total energy that
a body radiates.This is where luminosity comes to use while evaluating brightness of celestial
objects.
It is common knowledge that the more energy and light a star emits, the brighter that
star appears.
The use of luminosity comes at places when the amount of energy radiated is calculated.
Luminosity is calculated in two forms, the first being visible radiation, and the second
consisting of all forms of radiation, that is visible as well as invisible radiation.Luminosity
is the measure of total electromagnetic energy radiated by a body per unit time.
It is a distance dependent quantity.
The radiated electromagnetic energy from stars in all forms of radiation is calculated over
a period of time using a device known as bolometre and the most accurate measurement is taken
to determine the luminosity of the cosmological object being taken into account.
Simply said, luminosity decides the level of brightness of celestial objects.
Stars, like our sun, radiate heat and light in space.
Brightness of stars, nevertheless, ultimately depends upon its distance from the reference
frame, as it is a common knowledge to us.
What amount of light radiated by celestial objects reaches us, is decided by intensity.
Intensity, or energy flux is power radiated or reflected per unit area by a celestial
object.
It is the most important term in space astronomy which is used in measurement of brightness
of celestial objects.
In mathematical terms , Intensity is inversely proportional to square of distance of object
from reference frame and directly proportional to Luminosity of body.
Hence, Intensity of light of an object decreases with distance and increases with increasing
luminosity.
Remember the difference between luminosity and intensity.
Luminosity is time dependent, whereas intensity is distance dependent.
Stars like red supergiants nearing the end of their lives radiate more power than normal
main sequence stars, which makes them shine brighter than other stars.
Combined factors of luminosity, intensity and magnitude decides the brightness of stars.
On the other hand, brightness of planets is decided by a combined factors of atmospheric
conditions,distance from the observer's frame of reference,size of planet ,distance from
sun and visibility due to the sun's interference in observations.
The closest planet to earth is not venus, it’s mercury, yet venus appears brighter
than all other planets in the solar system.
If distance was the only deciding factor of brightness, mercury would have litten the
sky more than the moon does.
The major factor deciding the brightness of planets as seen from earth is thickness of
the planet’s atmosphere.
The composition of the atmosphere of a planet decides how much light the planet’s going
to reflect.Carbon dioxide tends to act as a good reflector of radiation.
Generally, planets with dense and thick atmospheres of carbon dioxide, like venus and jupiter
tend to reflect more than 60 percent of incoming visible radiation.
Saturn also reflects more than half of incoming radiation but it is not as bright as Jupiter
and Venus due to distance factor.
Hence, Venus and Jupiter tend to appear brighter as compared to other planets.
Mercury, having a thin atmosphere, tends to reflect only 10 percent of incoming radiation.
Another factor deciding the brightness of planets is its distance from the sun.
Planets closer to the sun tend to appear brighter than others because they receive more incoming
radiation than others.
Intensity becomes a deciding factor when differentiating on the basis of distance from the sun.
For example, Venus is eight times closer to the sun than Jupiter.
Hence, the intensity of radiation that Venus reflects back to us is far greater than what
Jupiter does, making it twice as bright as Jupiter.
Although the moon is a poor reflector of light due to absence of an atmosphere, it still
appears brighter than Venus and Jupiter due to its proximity from earth.
Size of a planet also decides the amount of radiation a planet reflects.
Larger planets, which have a greater surface area than other planets, tend to receive more
power than other planets.
However, Intensity of light received by us from a celestial body is inversely proportional
to the square of distance of the object.
Hence as the separation of the planet from our reference frame increases, its brightness
diminishes noticeably faster than usual.
However, Planets appear brighter than stars because of the large separation between us
and other stars in the universe.
The amount of radiation emitted by stars is far more than that reflected by Venus.
It’s just that the radiation and intensity of radiation dissipates throughout the cosmos
before it even reaches us due to the large distance between us and the stars.
Even the closest star to earth, the Proxima Centauri, lies more than 4 light years apart.
If it weren’t for the distance factor of stars, our night sky would not exist and the
brightness of Venus would have been nothing compared to that of other stars.
So next time you look at the night sky and observe planets like Venus and how bright
they are, remember this video.
So what do you think about these planets brighter than stars and even looking like one sometimes?
Let us know what you think in the comment section below and share the video with your
friends!