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3 Dec 2025
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Astrophysics: Exploring Orbits, Stars, and the Big Bang
Aliki ๐ฉต
@study_wabbit
1 / 3
Our Solar System and Gravity
Your neighbourhood in space is pretty incredible - our solar system contains 8 planets all dancing around the Sun in their own special orbits. The inner planets (Mercury, Venus, Earth, and Mars) are rocky and relatively small, whilst the outer planets (Jupiter, Saturn, Uranus, and Neptune) are massive gas giants made mostly of hydrogen and helium.
Here's something that'll blow your mind: weight and mass aren't the same thing! Your mass stays the same everywhere (measured in kg), but your weight changes depending on which planet you're on because weight is actually a force measured in Newtons.
The magic formula you need to remember is W = mg, where gravitational field strength on Earth is 10 N/kg. So if you have a mass of 50kg, you'd weigh 500N on Earth but much less on the Moon!
Quick Check: A 60kg astronaut would weigh 600N on Earth - can you work out their weight on a planet where g = 4 N/kg?
Orbits, Comets and Stellar Brightness
Comets are like cosmic snowballs made of ice and rock that zoom around the Sun in highly elliptical orbits. Their spectacular tails always point away from the Sun because that's where the melted ice gets blown by solar wind!
Different objects have completely different orbit shapes. Moons orbit in neat circles around planets, planets follow slightly elliptical paths around the Sun, but comets have wildly stretched orbits that bring them close to the Sun then fling them far out into space.
When you look up at stars, their brightness depends on two things: how much energy they're pumping out (luminosity) and how far away they are. The relationship is brightness = L/distanceยฒ, so doubling the distance makes a star four times dimmer!
Star colour tells you about temperature - blue stars are absolutely scorching (over 30,000K), whilst red stars are the coolest (under 3,700K). Our Sun sits in the middle as a yellow star at about 5,600K.
Fun Fact: The fastest comets can reach speeds of over 150,000 mph when they're closest to the Sun!
Star Life Cycles and the Big Bang
Stars have epic life stories that span billions of years! Every star begins as a nebula - a massive cloud of dust and gas that gravity pulls together. As it heats up, it becomes a protostar, then eventually a main sequence star when nuclear fusion kicks in properly.
When stars run out of hydrogen fuel, they expand into red giants or red supergiants. Smaller stars like our Sun will eventually become white dwarfs and fade away, but massive stars go out with a bang as supernovas and can become neutron stars or even black holes.
The Big Bang Theory explains how our universe started from a tiny point and has been expanding ever since. We know this is true because of red shift - light from distant galaxies is stretched towards longer, redder wavelengths as they move away from us.
Cosmic Microwave Background Radiation (CMBR) is like the universe's baby photo - it's the leftover radiation from the Big Bang that's been stretched from high-energy gamma rays into microwaves as space expanded.
Mind-Blowing Fact: The CMBR you can detect today has been travelling through space for nearly 14 billion years!
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Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
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Best app on earth! no words because itโs too good
Thomas R
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Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
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This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didnโt even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as Iโm sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH ๐๐๐ฒ๐ค๐โจ๐๐ฎ
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because itโs too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didnโt even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as Iโm sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH ๐๐๐ฒ๐ค๐โจ๐๐ฎ
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
Astrophysics: Exploring Orbits, Stars, and the Big Bang
Aliki ๐ฉต
@study_wabbit
Get ready to explore the mind-blowing world of astrophysics! From understanding why you weigh less on the Moon to discovering how stars are born and die, this topic covers everything from our solar system to the origins of the entire... Show more
Our Solar System and Gravity
Your neighbourhood in space is pretty incredible - our solar system contains 8 planets all dancing around the Sun in their own special orbits. The inner planets (Mercury, Venus, Earth, and Mars) are rocky and relatively small, whilst the outer planets (Jupiter, Saturn, Uranus, and Neptune) are massive gas giants made mostly of hydrogen and helium.
Here's something that'll blow your mind: weight and mass aren't the same thing! Your mass stays the same everywhere (measured in kg), but your weight changes depending on which planet you're on because weight is actually a force measured in Newtons.
The magic formula you need to remember is W = mg, where gravitational field strength on Earth is 10 N/kg. So if you have a mass of 50kg, you'd weigh 500N on Earth but much less on the Moon!
Quick Check: A 60kg astronaut would weigh 600N on Earth - can you work out their weight on a planet where g = 4 N/kg?
Sign up to see the contentIt's free!
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Orbits, Comets and Stellar Brightness
Comets are like cosmic snowballs made of ice and rock that zoom around the Sun in highly elliptical orbits. Their spectacular tails always point away from the Sun because that's where the melted ice gets blown by solar wind!
Different objects have completely different orbit shapes. Moons orbit in neat circles around planets, planets follow slightly elliptical paths around the Sun, but comets have wildly stretched orbits that bring them close to the Sun then fling them far out into space.
When you look up at stars, their brightness depends on two things: how much energy they're pumping out (luminosity) and how far away they are. The relationship is brightness = L/distanceยฒ, so doubling the distance makes a star four times dimmer!
Star colour tells you about temperature - blue stars are absolutely scorching (over 30,000K), whilst red stars are the coolest (under 3,700K). Our Sun sits in the middle as a yellow star at about 5,600K.
Fun Fact: The fastest comets can reach speeds of over 150,000 mph when they're closest to the Sun!
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Star Life Cycles and the Big Bang
Stars have epic life stories that span billions of years! Every star begins as a nebula - a massive cloud of dust and gas that gravity pulls together. As it heats up, it becomes a protostar, then eventually a main sequence star when nuclear fusion kicks in properly.
When stars run out of hydrogen fuel, they expand into red giants or red supergiants. Smaller stars like our Sun will eventually become white dwarfs and fade away, but massive stars go out with a bang as supernovas and can become neutron stars or even black holes.
The Big Bang Theory explains how our universe started from a tiny point and has been expanding ever since. We know this is true because of red shift - light from distant galaxies is stretched towards longer, redder wavelengths as they move away from us.
Cosmic Microwave Background Radiation (CMBR) is like the universe's baby photo - it's the leftover radiation from the Big Bang that's been stretched from high-energy gamma rays into microwaves as space expanded.
Mind-Blowing Fact: The CMBR you can detect today has been travelling through space for nearly 14 billion years!
We thought youโd never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help โ all at your fingertips.
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Most popular content in Physics
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English - inspector calls quotes and analysis
Quotes from every main character
English Literature10
Can't find what you're looking for? Explore other subjects.
Students love us โ and so will you.
4.9/5
App Store
4.8/5
Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because itโs too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didnโt even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as Iโm sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH ๐๐๐ฒ๐ค๐โจ๐๐ฎ
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because itโs too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didnโt even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as Iโm sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH ๐๐๐ฒ๐ค๐โจ๐๐ฎ
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user