Our Sun is shining because its core is fusing hydrogen nuclei together to form helium. It's a main sequence star, which means it's in hydrostatic equilibrium and the outward pressure from this nuclear fusion perfectly balances the gravitational forces trying to collapse the Sun in on itself.

But one day (about 5.4 billion years from now) our Sun will run out of hydrogen fuel in its core and only have helium at its centre. The Sun will stop counteracting gravity and contract slightly. 

When this contraction happens, the temperature will increase - and hydrogen fusion will start to happen in the shell around the helium core. This will cause the Sun to expand.

The outer layers of hydrogen will decrease in temperature, and this makes them look redder.

The Sun is now a red giant.

All main sequence stars between one-fifth and 10 times the mass of our Sun will become red giants when their hydrogen reserves run out in their core.

We're not sure. Scientists believe a red giant Sun will grow large enough to encompass the orbits of Mercury, Venus, and maybe even Earth.

Even if the Earth did survive, its going to be pretty close to the intense heat of the red giant Sun. The Earth's surface will, most likely, be scorched and inhospitable to life.

​The good news is that the Sun's expansion is predicted to alter the Earth's orbit - but the bad news is that we still won't escape a fiery death.

Why do Red Giants expand? You can find out more here. Here's a pretty big repository of red giant facts from Space.com. Red Giants could also explain the origins of the elements in our universe.

​And this is a pretty cool video from Dr. Mark Morris, a professor of astronomy at UCLA, covering the future of our Sun:

Twinkle, twinkle little star. You're not always what you are.

The "stars" you see at night may not be stars. Some of them are planets. Some of them are galaxies. But most of them are stars - and they're all completely unique.

But the human race does like to put things into boxes, so here are all the categories of early-life stars*:

A protostar is what you have before a star forms. It's the swirling collection of gas that's collapsing down into a star. It's getting hotter and denser - but nuclear fusion reactions haven't started yet.

Just before a star moves onto the main sequence, they're classified as a T Tauri. They look like a main sequence star and have the same temperature, but they're brighter because they're larger. The key difference is that a T Tauri generates its energy by gravitational pressure - not nuclear fusion.

The majority of the stars in our galaxy are main sequence stars. They're in a state called hydrostatic equilibrium. This means the gravity of the star that's pulling everything together is balanced by the light pressure from the nuclear fusion reactions pushing everything outwards. The result is that the star is spherical (or nearly spherical) in shape.

Main sequence stars are a pretty diverse bunch and can change in colour, size, brightness and mass but they all do the same thing: convert hydrogen to helium in their cores, a process called nuclear fusion, which releases a tremendous amount of energy.

Main sequence stars are classified by their luminosity (aka their brightness) into seven categories: O, B, A, F, G, K and M. O are the hottest and blue in colour, M are the coolest and read in colour.

Main sequence stars can be as small as 8% the mass of our Sun and can, theoretically, go up to 100 times the mass of the Sun.

Smaller bodies (less than 8% the mass of our Sun) do not have enough mass for nuclear fusion reactions to start. These brown dwarfs are too small to be stars and too big to be planets - and they never twinkle.

Next week, I'll look at what happens at the end of a star's life...

This is a pretty thorough description of the different classifications of main sequence star. Here's some more information on brown dwarfs.

This post goes into a little more depth around main sequence stars and here's some more information on the nuclear fusion reactions in stars that keep them shining.

* By early-life I mean those stars that are about to, may never, or have achieved the nuclear fusion of hydrogen to helium.

​Hello. I’m the freelance writer who gets tech. I have two degrees in Physics and, during my studies, I became increasingly frustrated with the complicated language used to describe some outstanding scientific principles. Language should aid our understanding — in science, it often feels like a barrier.
​
So, I want to simplify these science sayings and this blog series “Sunday Science” gives a quick, no-nonsense definition of the complex-sounding scientific terms you often hear, but may not completely understand. 

If there’s a scientific term or topic you’d like me to tackle in my next post, fire an email to [email protected] or leave a comment below. If you want to sign up to our weekly newsletter, click here.

Rockets are used to transport objects and people into space. That's a really dull sentence to describe potentially the most EXCITING engineering achievement of the human race.

But how do they work? Well, there are four basic forces of flight: lift, gravity, thrust and drag. Thrust and lift are positive forces that propel a rocket into space. Gravity and drag are negative forces that slow a rocket down.

It's a bit like letting air out of a balloon. The weight of the balloon tries to pull it down to Earth (gravity) and when it's flying around the room, air resistance (drag) pulls the balloon in the opposite direction of its movement.

The air whooshing out of the balloon provides the thrust it needs to oppose its weight. The lift is a force at right angles to the thrust, which stabilises and controls the direction of flight. 

Rockets don't have air. They carry a lot of fuel to get them into space. When this liquid fuel burns, it produces gas. The build up of this exhaust gas escapes the rocket with a lot of force and provides enough thrust for the rocket to blast off.

Rockets need a huge amount of energy to overcome gravity and stop them falling back to Earth. In his famous 1962 speech championing travel to the Moon, US President John F. Kennedy compared the power of a rocket to "10,000 automobiles with their accelerators on the floor." 

To give you a more exact figure, a rocket needs to achieve speeds of 25,000 mph to escape the Earth's gravity. 

Modern rockets are made up of several parts. The main parts are the propulsion system (to get the rocket into space), payload system (where the astronauts sit) and guidance system (to manoeuvre the rocket), but they can contain around three million different parts. ​They all look different too, but here's a basic picture of a rocket, courtesy of NASA:

This is a more thorough (but not overly complicated) explanation on how rockets work and here's some more information on their structure. If you want to know a little bit more about how Ironman flies, check out this awesome explanation.

I've distilled a really interesting topic into a short post. For example, there's been a lot written about the space race between the Russians and Americans, but I'd hardheartedly recommend you read Hidden Figures: The Story of the African-American Women Who Helped Win The Space Race. Also, did you know that the first true rocket was used in 1232? The history of the rocket is a fascinating tale that goes beyond the space race - you can read more here and here. Also, check out this in-depth explanation of the final stages before blast off!

A rocket launch is a pretty spectacular sight. Speaking last week, Tim Peake said: “I was mesmerised by the noise and the power of the rocket launch when I first saw one.” So, here's a launch in action - make sure you turn up the volume to get an idea of how LOUD one can be!

​Could we colonise Mars by 2024? Elon Musk seems to think so and astronaut Tim Peake predicted a similar timescale for a trip to the Red Planet when he spoke at York University, a couple of days before Musk's prediction was made.

Echoing Apollo astronaut Charlie Duke's optimism around a manned mission to Mars, Peake predicts we will see humans on Mars by the late 2030s "but no one nation has the resources to do this. If we can include private investment and companies like SpaceX this could speed things up," he added.

Peake was a little more considered in his predictions than Musk. "People are always over ambitious when talking about space exploration. It is hard and difficult work," he said.

"One would like to think we would have permanent occupancy modules on Mars in 100 years," according to Peake.

Tim Peake shared a lot of nuggets of information about space exploration, living in space and the importance of snorkels and nappies during a space walk (yes, really). Here are my 12 favourite titbits from his fascinating talk:

Peake flew from Russia’s Baikonur Cosmodrome in Kazakhstan, where a whole other set of prelaunch traditions are in place.

Most of these rituals pay tribute to the first human to go into space, Yuri Gagarin, including peeing on the back tyre of the bus that takes cosmonauts to the launch pad, just as Gagarin did in 1961. According to Tim, a space suit is not the easiest item of clothing to undo and do up again.

​Tim traveled to the Space Station with ​Veteran Russian commander Yuri Malenchenko and NASA astronaut Tim Kopra in the Soyuz capsule.

The Soyuz capsule had to attach to the Space Station but the automatic docking system failed to operate, so manual control had to be taken.

It sounded like a pretty hairy time as the optics on the capsule were flooded with light as the transition from day to night occurred when the automatic docking system failed.

Malenchenko took control and eventually docked the craft. "He knew the danger and waited until better conditions," Peake added.

When Soyuz's crew met the crew at the Space Station, their physical and mental condition was "great", according to Tim, despite spending nine-months in space.

The working week on the Space Station is also your (not so typical) 9 to 5. So, it is structured and, at times, quite solitary according to Peake as each crew member works on different scientific experiments.

“No two days are the same and that's what makes it so exciting," according to Peake, who also took remote control of a Mars rover (which was based on Earth at the time) while on the Space Station. It was an important proof of concept for future exploratory missions to the Red Planet.

Tim and the crew also had to clean the Space Station every Saturday (do astronauts have feather dusters? I'd like to think so). Tim also said the station had a "noisy party atmosphere" because of all the different instruments left to run, which mean there's a constant 40-50 decibels of noise.

"It also smelt of a laboratory," Tim added. But you soon get used to the smell.

Tim conducted one space walk during his time on the Space Station to fix a faulty electrical box. Peake said this was "the moment of greatest apprehension because you do not know what that moment will feel like."

Dropping out of the airlock, Peake said his primary concern was getting tangled in the tethers on his suit and "the second thing was not to look down as it's an awfully long way down at 400km above the Earth."

The space walk was running 10 minutes ahead of schedule, so Tim had to wait for the Sun to set and the power supply to be cut before the repair work to the electrical box could be carried out. "Mission control said hang out. I don't think anyone else has been told to hang out in space. It was one of the most remarkable moments of my life," according to Peake.

"I felt myself completely immersed in space and had a quiet reflection on where we were and what we were doing."

The CO2 sensor for Tim's fellow spacewalker, Tim Kopra, went off during the excursion, which usually signals a water leak in the suit.

"We have a procedure to deal with this," according to Peake, "A snorkel so you can still breathe and a nappy at the back of the helmet to absorb moisture."

All that scientific knowledge and development and "we came up with a snorkel and a nappy," Tim added.

The spacewalk was terminated early as a result, but the astronauts were outside the Space Station for 4 hours and 43 minutes.

When Tim did have a spare few minutes (he also completed a huge amount of outreach work), he photographed the mind-blowing views from the Space Station. Check out his stunning book of those views (plus, all Tim's proceeds go to the Prince's Trust).

"Watching Earth from space is mesmerising and it's constantly changing. During the day, you do not see borders or cities, it's all about the mountains and glaciers. Earth is a wonderful geological feature in the making. At night, the Earth comes alive," Peake added.

Reentry sounds like quite a traumatic experience. Tim described the experience as if "the capsule is blowing itself apart."

And, when the breaking parachutes slowly open, "it was 20 seconds of the most crazy roller coaster ride of my life," according to Peake. "You don't want to have your tongue between your teeth. You're bracing for something that is essentially a car crash," he added. Yikes.

Readjusting to life on Earth isn't easy. Peake found he was dizzy, disoriented and felt pretty rough for the first three days on Earth. Muscle distribution is something that really suffers, according to Peake, and he is only just starting to fully recover his bone density.

I don't know about you, but the thought of going into space terrifies me. Peake, on the other hand, was quietly cautious when talking about the launch process.

"You deal with the consequences of what you are doing long before you go on the rocket," he said. "No one should happily sit on 300 tonnes of rocket fuel - if you can then you may not be psychologically prepared to go to space."

Peake's time on the Space Station has changed his perspective on our planet. "You feel like you know the Earth pretty well, even though there are areas you have never been to, or are likely to visit."

This "overview effect" is a psychological phenomenon shared by many astronauts.

While there's no gravity on the Space Station, there is a convention of what's up and what's down. It's also important to be a "good crew member" when it comes to personal hygiene on the Space Station. Although your time there means "it's the best pedicure you'll ever have", you have to be careful when you whip your socks off, according to Peake.

And the astronauts returned with "lizard feet" as they hooked their feet to steady themselves as they moved around in zero gravity, which meant they rubbed in unusual places.

It looks like Tim will have to cope with lizard feet at least one more time too, as he's due to go back up to the Space Station. And he also has high hopes for his future in space, as he added:

With only six moonwalkers now left on Earth, fingers crossed for Tim - and the future of space exploration, as we know it.

SETI is a rather simple acronym that deals with one of humanity's biggest questions: are we alone in the universe?

The Search for Extraterrestrial Intelligence (SETI) is looking for advanced civilisations. It's not interested in tiny microbes or any such simplistic lifeforms.

Most SETI searches hunt for radio and optical signals to indicate intelligent life. 

Radio astronomy is used to hunt for radio signals that an intelligent civilisation could produce. These tend to look for narrow-band signals, which are radio emissions that only cover a tiny part of the radio spectrum. This is because natural objects will emit radio waves across the spectrum, but if you find a signal that just uses a small region of the radio spectrum, it could have an artificial source. Scientists also use optical searches to look for brief flashes of light in the search for intelligent life.

​Such optical and radio signals could be deliberately beamed out of a planet, or they could be picked up accidentally. Earth has unintentionally broadcast radio and radar signals since World War 2. We also purposefully transmitted a simple message from the Arecibo Observatory in Puerto Rico in 1974.

This radio message (shown above) carried basic information about the human race and was fired at the globular star cluster M13 in the hope that extraterrestrial intelligence might receive and decipher it. 

The SETI Institute is the largest player in the hunt for advanced civilisations in our cosmos.

Set up in 1988, it is now a not-for-profit organisation (after its funding was withdrawn after a year) and is made up of scientists, engineers, teachers and other staff.

The Institute has more than 100 active projects. And, in a joint project with the University of California, Berkeley it built the Allen Telescope Array - a 42-strong radio telescope array to examine one million stars in the next two decades.

Despite hopes being raised just over a year ago, SETI hasn't found any evidence of intelligent extraterrestrial life.

We'll just have to wait a little bit longer for ET to phone home.

There's so much information on the SETI Institute's website and the FAQ section is particularly useful. 

You can also analyse the light from an exoplanet (a planet outside of our own Solar System) to work out what its atmosphere is made up of. If the atmosphere is made up of oxygen, nitrous oxide and methane, then this could indicate life is present. If you'd like to find out more about exoplanets - NASA's Kepler space telescope has found dozens of potentially habitable worlds, including this mysterious "alien megastructure".

And here's a list of the seven most likely places in the universe where intelligent life could exist.

​Want more Sunday Science? I've just started the Sunday Science Facebook Group. If you've got a question about science, want to see more science on your news feed or just want to keep up to date with the latest on the blog, it would be great to welcome you to the group!

​Hello. I’m the freelance writer who gets tech. I have two degrees in Physics and, during my studies, I became increasingly frustrated with the complicated language used to describe some outstanding scientific principles. Language should aid our understanding — in science, it often feels like a barrier.
​
So, I want to simplify these science sayings and this blog series “Sunday Science” gives a quick, no-nonsense definition of the complex-sounding scientific terms you often hear, but may not completely understand. 

If there’s a scientific term or topic you’d like me to tackle in my next post, fire an email to [email protected] or leave a comment below. If you want to sign up to our weekly newsletter, click here.