Space and the Universe

One number decides everything

The life cycle of stars usually arrives as a flowchart with a fork in it. A cloud of gas, then a star, then the fork, and two different endings depending on which way you go. Revision then becomes an attempt to remember which branch leads where. ⭐⭐ THERE IS NOTHING TO REMEMBER, BECAUSE ONE NUMBER DECIDES A STAR'S ENTIRE BIOGRAPHY: HOW MUCH MATERIAL IT HAD AT THE START. How long it lives. How it behaves when its fuel runs low. How it ends. What it leaves behind. All of it follows from that one quantity, which is why the fork exists at all. ⭐ AND THE REASON IS A SINGLE BALANCE THAT RUNS THROUGH THIS WHOLE TOPIC: GRAVITY PULLS INWARD, AND SOMETHING HAS TO PUSH BACK, OR THE THING COLLAPSES. Start with a cloud of gas and dust drifting in space. Gravity pulls it together, slowly at first and then faster as it gets closer in, and the middle gets denser and hotter as it is squeezed. ⭐ A STAR BEGINS AT THE MOMENT THE SQUEEZING IS HARD ENOUGH FOR FUSION TO START IN THE CENTRE. THAT IS THE OUTWARD PUSH ARRIVING, AND IT IS WHAT STOPS THE COLLAPSE. From then on the star is a standoff. Gravity pulling in, the energy released in the middle pushing out, and the two in balance. That balance is the long steady middle of a star's life, and while it holds, very little changes. ⭐⭐ NOW THE PART THAT SOUNDS BACKWARDS AND IS THE BEST TEST OF WHETHER YOU HAVE UNDERSTOOD IT. THE BIGGEST STARS HAVE THE SHORTEST LIVES. More material means a stronger pull. A stronger pull has to be matched by a stronger push. So a very massive star has to burn through its fuel at an enormous rate simply to hold itself up, and having more fuel does not save it, because it is spending it far faster than it gained it. ⚠️ A STAR IS NOT USING FUEL TO SHINE. IT IS USING FUEL TO AVOID COLLAPSING, AND THE SHINING IS WHAT THAT LOOKS LIKE FROM OUTSIDE. And when the fuel runs low, the push weakens while the pull does not, so gravity begins to win. What happens then depends entirely on how hard that pull is, which is the mass again. ⭐ THE LAST PART OF THIS TOPIC ASKS THE SAME QUESTION ABOUT EVERYTHING AT ONCE. The universe is observed to be expanding, which makes it the one place in this topic where gravity is not winning, and that observation is where the account of its origin comes from. Carry one question through everything that follows: what is gravity pulling on, and what is pushing back?

Pull, push, and who is winning

The same balance at three scales. Notice that only the first column has a settled answer.

Tap the two about the balance

Tap the TWO statements that describe something pushing back against gravity.

• Energy released by fusion in the centre of a star holds the star up against its own weight
• A star begins when squeezing has gone far enough for fusion to start, which is what stops the collapse
• A planet stays in orbit because it is moving sideways fast enough to keep missing what it is falling towards
• A cloud of gas and dust is pulled together, and its middle grows denser and hotter

Why the biggest die youngest

A very massive star has far more fuel than a small one, and it runs out far sooner. Why?

• Because more mass means a stronger inward pull, which has to be matched by a stronger outward push, so the star burns through its fuel at an enormous rate simply to avoid collapsing
• Because massive stars actually start with less fuel than small ones
• Because large stars lose most of their fuel to space before they can use it
• Because they are hotter, and hotter things always use energy faster

Five terms for this topic

Five terms, each defined by what it is. The first two are the balance; the last two are an observation and the account built on it.

Match each stage to what is happening

Two that follow from the balance

Select the TWO statements that follow from a star being a standoff between gravity and an outward push.

• A star's mass at the start decides how long it lives and how it ends, so the fork in the life cycle is not arbitrary
• A star spends fuel to avoid collapsing rather than to shine, and the shining is what that looks like from outside
• A planet stays in orbit because an outward force balances gravity
• All stars end in the same way once their fuel is gone

Keeping observation apart from conclusion

Two routines for this topic, because it asks two different kinds of question. ⭐⭐ FOR ANYTHING ABOUT A STAR, ASK WHAT IS PULLING AND WHAT IS PUSHING BACK, AND THEN ASK WHICH IS WINNING. That one pair of questions gets you through the whole life cycle without a flowchart, and it tells you why each stage follows the one before. AND IF THE QUESTION IS ABOUT HOW LONG OR HOW IT ENDS, GO STRAIGHT TO THE MASS. ⭐ THE MASS IS THE ANSWER TO BOTH, AND SAYING SO EXPLICITLY IS WHAT TURNS A DESCRIPTION INTO AN EXPLANATION. ⭐⭐ FOR ANYTHING ABOUT THE ORIGIN OF THE UNIVERSE, THE ROUTINE IS DIFFERENT, AND IT IS THE ONE THAT EARNS THE MARKS: SEPARATE WHAT WAS OBSERVED FROM WHAT WAS CONCLUDED. Say what is actually seen. Then say what that is taken to mean. Then say why that conclusion follows rather than some other one. ⚠️ AN ANSWER THAT RUNS THE THREE TOGETHER READS AS THOUGH THE CONCLUSION WERE SOMETHING ANYBODY COULD SEE DIRECTLY, AND NOBODY CAN. ⚠️ THREE HABITS THAT COST MARKS HERE. ⚠️ THE FIRST IS TREATING AN ORBIT AS A BALANCE OF FORCES. ⭐ NOTHING PUSHES OUTWARD ON AN ORBITING PLANET. IT KEEPS MISSING WHAT IT IS FALLING TOWARDS, AND ANSWERS THAT INVENT AN OUTWARD FORCE ARE MARKED WRONG EVEN THOUGH THE PICTURE THEY DESCRIBE LOOKS RIGHT. ⚠️ THE SECOND IS SAYING A STAR SHINES BECAUSE IT IS BURNING FUEL, AND STOPPING THERE. It is closer to say it is spending fuel to hold itself up. The difference matters because only the second version explains the lifetimes. ⚠️ AND THE THIRD IS DESCRIBING THE ORIGIN OF THE UNIVERSE AS THOUGH IT HAD BEEN WATCHED. ⭐ IT IS AN ACCOUNT BUILT BACKWARDS FROM WHAT IS SEEN NOW, AND AN ANSWER THAT SAYS SO IS DOING BETTER SCIENCE THAN ONE THAT NARRATES IT. One last thing. Where you are not sure of a number, leave it out. Every argument in this topic can be made in terms of more, less, longer and shorter, and an answer built that way cannot be wrong about a figure it never gave.

Two stars, and which one lasts

Take two stars that formed at the same moment out of the same cloud. One gathered a great deal of material. The other gathered very little. Neither is a real star and no numbers are attached, because the argument does not need any. Ask which one is still there much later, and almost everybody picks the big one. It has more to burn, so it ought to last longer. ⚠️⚠️ IT IS THE FIRST TO GO, AND BY AN ENORMOUS MARGIN. THE BIG ONE IS LONG FINISHED WHILE THE SMALL ONE IS STILL GOING QUIETLY ON. ⭐ THE MISTAKE IS TREATING THE FUEL AS A TANK AND ASKING HOW FULL IT IS. THE THING THAT MATTERS IS NOT HOW MUCH THERE IS BUT HOW FAST IT HAS TO BE SPENT, AND THAT IS NOT A CHOICE THE STAR MAKES. Think about what the big one has to do just to stay the shape it is. Every part of it is being pulled towards the middle, and there is a great deal of it, so the pull in the centre is fierce. To avoid being crushed, the centre has to push back just as hard, every second, without pausing. ⭐⭐ AND THE ONLY WAY TO PUSH THAT HARD IS TO SPEND FUEL AT A TREMENDOUS RATE. THE STAR IS NOT CHOOSING TO BURN BRIGHTLY. IT IS PAYING WHAT IT COSTS NOT TO COLLAPSE. Now the small one. Very little material, so a gentle pull, so only a gentle push is needed to hold it up. It spends its fuel slowly because slowly is all it needs. ⭐ SO THE BIG STAR STARTED WITH MORE AND IS SPENDING IT VASTLY FASTER, AND THE SECOND FACT OVERWHELMS THE FIRST. A larger tank emptied through a much wider opening runs dry first, and it is not close. ⭐⭐ THE GENERAL LESSON IS WORTH MORE THAN THE EXAMPLE: WHEN SOMETHING HAS BOTH A LARGER SUPPLY AND A HIGHER RATE OF USE, THE RATE USUALLY DECIDES THE OUTCOME. And notice the sentence that carries the whole thing: what a star has to do is not shine. It is not fall in. The shining is simply how that looks from here.

Order a star's life by the balance

Put these six stages in order, from a drifting cloud to the point where the mass decides the ending.

• A cloud of gas and dust drifts in space, with nothing yet holding it apart
• Gravity pulls it together, and the middle grows denser and hotter as it is squeezed
• The squeezing is hard enough for fusion to start, and the outward push arrives
• Push and pull match, and the star is stable for most of its life
• The fuel runs low, so the push weakens while the pull does not
• Gravity begins to win, and how strong that pull is decides what the star becomes

Assemble the sentence about lifetime

This is the sentence that explains a star's lifetime. Assemble it.

What was seen, and what was concluded

A student writes that scientists observed the universe expanding. What needs correcting?

• What is observed is that light from distant galaxies is shifted towards the red end. The expansion is the conclusion drawn from that observation, and keeping the two apart is what the question is testing
• Nothing: the expansion has been observed directly through telescopes
• That the universe is not in fact expanding
• That the light is shifted towards the blue end rather than the red

The balance run

Five questions on pull, push and what follows. Three lives.

Complete the space facts

The attraction every mass has for every other mass, which pulls matter together, is _____. The joining of small nuclei in the centre of a star, releasing the energy that holds it up, is _____. The shift of light from a distant galaxy towards the red end of the spectrum is _____. The scientific account in which the universe began in an extremely hot, dense state and has been expanding since is the _____.

Three answers about the evidence

Three students write about the origin of the universe. Nothing they say is false; the reasoning is what needs work.

• A student writes that scientists watched the universe expand and worked backwards to the Big Bang. How would you repair it?
• An answer says a star shines because it is burning fuel, and then cannot explain why the most massive stars have the shortest lives. What is the missing idea?
• A student argues that because the universe is expanding, gravity must have stopped working at large scales. Is that the strongest version?

Explain a star's life by the balance

A friend has a flowchart of the life cycle of stars and cannot remember which branch is which. Explain how the whole thing follows from one balance, so that they do not need the flowchart.

• Name the two things in the balance, and say which way each one acts
• Explain how a star begins, in terms of when the outward push first arrives
• Explain what the long stable middle of a star's life is, in terms of the balance
• Explain why a more massive star has a shorter life, using the rate at which it must spend fuel
• Say which single quantity decides how a star ends, and why it is that one
• Finish by explaining why an orbit is the odd one out in this topic
• Keep to more, less, longer and shorter, and give no figure you are not certain of