Space Physics
IGCSE Physics Topic 6 16:59 English narration · English + 中文 subtitles burned in
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Look up on a clear night, and you might see a few thousand stars.
在晴朗的夜晚抬头看，你也许能看到几千颗星星。
But that is almost nothing.
但这几乎什么都不算。
Our Sun is just one star among a hundred billion in our galaxy — and our galaxy is just one among billions more.
我们的太阳只是银河系里一千亿颗恒星中的一颗——而我们的银河系， 又只是数十亿个星系中的一个。
Space is almost unimaginably vast.
宇宙浩瀚得几乎无法想象。
And yet it all obeys the very same physics you have been learning.
然而这一切，都遵循着你一直在学的同样的物理。
This is space physics — from the Moon in our sky to the edge of the known Universe.
这就是空间物理学——从我们天空中的月亮，到已知宇宙的边缘。
Let's take the journey outward.
让我们一路向外，展开这段旅程。
Let's begin.
让我们开始吧。
Let's start close to home.
让我们从离家最近的地方开始。
The Earth spins — it rotates — on its tilted axis once a day — that turning gives us day and night, and the tilt gives us our seasons.
地球绕着它倾斜的地轴每天自转一圈—— 这种转动带来白天和黑夜，而这个倾斜带来了四季。
The Earth also orbits the Sun once a year, held in its path by gravity.
地球还每年绕太阳公转一圈， 靠引力维持在它的轨道上。
And the Moon orbits the Earth about once a month; as it goes round, we see different amounts of its sunlit half — the phases of the Moon.
而月球大约每月绕地球一圈；随着它的运行， 我们看到它被太阳照亮的那一半的不同部分——这就是月相。
So why do we get seasons?
那么为什么会有四季？
Not because the Earth gets closer to the Sun.
不是因为地球离太阳更近了。
The reason is the tilt of its axis.
原因是地轴的倾斜。
The axis keeps pointing the same way all year, so for one half of the orbit the northern half of the Earth leans towards the Sun.
地轴全年都指向同一个方向，所以在轨道的一半时间里，地球的北半部朝太阳倾斜。
Sunlight hits it more directly, and for longer each day, and that half has summer.
阳光照得更直，每天照的时间也更长，这一半就是夏天。
At the same time the southern half leans away, and has winter.
同时，南半部背离太阳，是冬天。
Six months later, they swap.
六个月以后，两者交换。
The Moon makes no light of its own.
月球自己不发光。
It reflects sunlight, so the half facing the Sun is always lit.
它只是反射太阳光，所以朝着太阳的那一半总是被照亮的。
As the Moon travels round the Earth, we see that lit half from different sides.
当月球绕着地球运行时，我们从不同的角度看到那被照亮的一半。
With the Moon between us and the Sun, the lit side faces away from us: a new moon.
当月球在我们和太阳之间时，被照亮的一面背对我们：这是新月。
When the Moon is on the far side, we see the whole lit face: a full moon.
当月球在另一侧时，我们看到整个被照亮的面：这是满月。
In between, we see a half moon.
在两者之间，我们看到半月。
One trip round takes about a month.
绕一圈大约要一个月。
Three motions, three times, and the exam likes to mix them up.
三种运动，三个时间，而考试很喜欢把它们弄混。
The Earth's spin on its own axis gives one day, about twenty-four hours — and it is that same spin that makes the Sun appear to move across the sky.
地球绕自己的地轴自转， 给出一天，大约二十四小时——也正是这个自转，让太阳看起来每天在天空中移动。
The Moon's orbit of the Earth gives one month.
月球绕地球公转，给出一个月。
The Earth's orbit of the Sun gives one year, about three hundred and sixty-five days.
地球绕太阳公转，给出一年，大约三百六十五天。
So from shortest to longest: one day, then one month, then one year.
所以从短到长依次是：一天，然后一个月，然后一年。
Anything moving in a circular orbit has an orbital speed.
任何做圆周轨道运动的物体都有一个轨道速度。
In one full orbit it travels the whole way round the circle — that distance is two pi times the radius — in a time equal to its orbital period.
绕行一整圈，它走过整个圆周—— 这段距离是二派乘以半径——所用的时间等于它的轨道周期。
So the speed is two pi times the radius, divided by the period.
所以速度就是二派乘以半径， 再除以周期。
Take a satellite orbiting at a radius of seven thousand kilometres, once every ninety minutes: its speed works out to about eight kilometres every second.
以一颗卫星为例，它在七千千米的半径上、每九十分钟绕一圈： 算出来它的速度大约是每秒八千米。
This is the International Space Station, falling around the Earth in a circle.
这是国际空间站，它正沿着一个圆形绕地球下落。
Its orbital speed is exactly what keeps it there: too slow, and it would come down; too fast, and it would fly away.
正是它的轨道速度让它留在那里：太慢，它会掉下来；太快，它会飞走。
And the same equation works for anything in orbit — a planet round the Sun, a moon round a planet, or a satellite round the Earth.
而同一个公式适用于任何在轨道上运行的物体——绕太阳的行星， 绕行星的卫星，或者绕地球的人造卫星。
Now the exam version, carefully.
现在按考试的方式仔细做一遍。
A space station orbits at a radius of seven thousand kilometres, and one orbit takes five thousand eight hundred seconds.
一个空间站在七千千米的半径上运行， 绕一圈用五千八百秒。
First, change the radius into metres: seven thousand kilometres is seven million metres.
第一步，把半径换成米：七千千米就是七百万米。
Now put the numbers in: two pi times seven million, all divided by five thousand eight hundred.
接着把数字代进去：二派乘以七百万，再全部除以五千八百。
That gives an orbital speed of about seven thousand six hundred metres per second.
算出来的轨道速度大约是每秒七千六百米。
Our Solar System has eight planets orbiting the Sun.
我们的太阳系有八颗行星绕着太阳运行。
The four small, rocky planets huddle close in; the four giants of gas and ice lie far out.
四颗又小又岩石的行星紧挨在里侧； 四颗气态和冰态的巨行星远在外侧。
Gravity holds them all in their orbits — and the further out a planet is, the slower it travels and the longer its year.
引力把它们都维持在各自的轨道上—— 行星越靠外，走得越慢，一年也越长。
Distances here are so huge that we measure them by how long light takes to cross them: sunlight takes eight minutes to reach the Earth.
这里的距离太大了， 我们用光穿过它们所需的时间来量：太阳光要走八分钟才能到达地球。
The whole system formed billions of years ago, from a spinning cloud of gas and dust pulled together by gravity.
整个太阳系形成于几十亿年前，来自一团被引力聚拢的、旋转的气体和尘埃云。
The Sun sits at the centre, and eight planets orbit it.
太阳位于中心，八颗行星绕着它运行。
Learn the order going outwards.
要把由内到外的顺序记住。
Mercury is closest, then Venus, then Earth, then Mars.
水星最近，然后是金星、地球、火星。
After Mars comes Jupiter, then Saturn, then Uranus, and Neptune is furthest out.
火星之后是木星，然后是土星、天王星， 最外面是海王星。
One sentence keeps them all in order: My Very Easy Method Just Speeds Up Naming Planets.
有一句英文句子能帮你记住它们的顺序： My Very Easy Method Just Speeds Up Naming Planets。
The eight planets fall into two families.
八颗行星分成两个家族。
The four nearest the Sun are small and rocky, with solid ground you could stand on: Mercury, Venus, Earth and Mars.
离太阳最近的四颗又小又是岩石的，有可以站上去的固体地面： 水星、金星、地球和火星。
The four furthest out are large and gaseous.
最外面的四颗又大又是气态的。
They are made mostly of gas, so there is no solid surface at all.
它们主要由气体组成，所以根本没有固体表面。
Jupiter and Saturn are the gas giants.
木星和土星是气态巨行星。
Uranus and Neptune hold far more ice, so we call them ice giants instead.
天王星和海王星含有多得多的冰，所以我们把它们叫做冰巨行星。
Here are three of them for real.
来看三颗真实的行星。
Mars is rocky and red, coloured by iron-rich dust, and small enough to walk on.
火星是岩石的、红色的，颜色来自富含铁的尘土， 而且小到可以在上面行走。
Jupiter is the giant: those brown and white stripes are bands of cloud in a deep atmosphere of gas.
木星是巨人：那些棕白色的条纹， 是它深厚气体大气里的云带。
Saturn is gas as well, wrapped in a wide flat ring of ice and dust.
土星也是气态的，外面裹着又宽又平的冰和尘埃环。
Out there, there is nothing solid to land on.
在那么远的地方，没有任何固体可以着陆。
Planets are not the only things going round the Sun.
绕着太阳运行的并不只有行星。
Pluto is a dwarf planet: it orbits the Sun, but it is too small to count as a planet.
冥王星是一颗矮行星：它绕太阳运行， 但太小了，算不上行星。
Asteroids are lumps of rock, and most of them lie in the asteroid belt between Mars and Jupiter.
小行星是一块块的岩石，大多数位于火星和木星之间的小行星带里。
Moons are natural satellites: they orbit a planet, not the Sun.
月球这样的天体是天然卫星：它们绕行星运行，而不是绕太阳。
And comets are icy bodies on long, stretched orbits.
而彗星是冰质天体，走在又长又扁的轨道上。
What holds the whole system together?
是什么把整个系统维系在一起？
The Sun holds almost all of the mass of the Solar System.
太阳占据了太阳系几乎全部的质量。
Its gravity reaches out across empty space and pulls on every planet, and that pull is what keeps each planet in its orbit.
它的引力跨过空旷的空间，拉住每一颗行星，正是这个拉力让每颗行星保持在自己的轨道上。
Gravitational field strength tells you how strong the pull is.
重力场强度告诉你这个拉力有多强。
It is bigger for a body with more mass, and it gets weaker the further away you go.
质量越大的天体，它越大； 而离得越远，它就越弱。
So the outer planets feel a weaker pull, and they move more slowly than the inner ones.
所以外侧的行星受到的拉力更弱，走得也比内侧的行星更慢。
Real orbits are not perfect circles.
真实的轨道不是完美的圆。
Most are ellipses, which are stretched circles, and the Sun is not at the centre of one.
大多数是椭圆，也就是被拉长的圆， 而太阳并不在椭圆的中心。
It sits at one focus, off to one side.
它位于其中一个焦点上，偏在一侧。
A comet has a very stretched orbit indeed.
彗星的轨道就被拉得非常长。
Now the part that earns marks: a body moves faster when it is closer to the Sun.
现在是能得分的部分： 物体离太阳越近，走得越快。
We explain it with the conservation of energy: the total stays the same, so coming inwards its gravitational potential energy falls, its kinetic energy rises, and it speeds up.
能量是守恒的，所以往里走时， 它的重力势能减少，动能增加，于是加速。
Going back out, the swap runs the other way and it slows down.
往外走时，交换反过来，它就减速。
Space is so big that we time journeys with light.
空间太大了，所以我们常用光来计时。
Light travels at three hundred million metres per second, and nothing goes faster.
光的速度是每秒三亿米，没有东西比它更快。
Time equals distance divided by speed.
时间等于距离除以速度。
The Sun is about one hundred and fifty billion metres away, so divide that distance by three hundred million.
太阳大约在一千五百亿米之外， 所以把这个距离除以三亿。
The answer is five hundred seconds, which is a bit over eight minutes.
答案是五百秒，也就是八分钟多一点。
The sunlight on your face left the Sun eight minutes ago.
照在你脸上的阳光，是八分钟前离开太阳的。
So where did all of it come from?
那么这一切是从哪里来的？
This is the accretion model.
这就是吸积模型。
It began as a giant cloud of gas and dust, containing many chemical elements.
它一开始是一团巨大的气体和尘埃云，里面含有许多化学元素。
Gravity pulled the cloud together.
引力把这团云聚拢起来。
The cloud was already spinning, so as it shrank it flattened into a spinning accretion disc.
这团云本来就在旋转，所以在收缩时被压扁成一个旋转的吸积盘。
Most of the matter fell to the middle and became the Sun, and the planets grew slowly from the material left over in the disc.
大部分物质落到中间，成为太阳，而行星则慢慢地由盘里剩下的物质长成。
And this is not just a theory on paper.
而这并不只是纸上的理论。
This is a real accretion disc, photographed around a young star called HL Tauri.
这是一个真实的吸积盘， 拍摄于一颗叫做金牛座HL的年轻恒星周围。
The bright rings are dust and gas still going round it, and the dark gaps are where new planets have already swept the dust up.
明亮的环是仍在绕行的尘埃和气体， 而暗的缝隙，就是新行星已经把尘埃扫干净的地方。
Our own Solar System looked like this once.
我们自己的太阳系，曾经也是这个样子。
That one disc also explains the two families.
这一个盘也解释了那两个家族。
Close to the young Sun it was hot.
在年轻的太阳附近，温度很高。
Only rock and metal could stay solid there, so the inner planets are small and rocky.
在那里只有岩石和金属能保持固态，所以内侧的行星又小又是岩石的。
Far out it was cold, so ices and gases could collect as well as rock.
在很远的外侧温度很低，所以除了岩石，冰和气体也能聚集起来。
There was far more material to build with, and those planets grew into the giants.
那里可用的物质多得多，于是那些行星长成了巨行星。
The Sun is a star — a giant ball of hot gas.
太阳是一颗恒星——一个巨大的高温气体球。
Deep in its core, nuclear fusion joins hydrogen into helium, releasing the light and heat that power life on Earth.
在它的核心深处，核聚变把氢聚合成氦， 释放出维持地球生命的光和热。
Our Sun is just one of a hundred billion stars in the Milky Way galaxy, and the Universe holds billions of galaxies.
我们的太阳只是银河系里一千亿颗恒星中的一颗， 而宇宙里有数十亿个星系。
Between the stars, distances are so vast we measure them in light-years — the distance light travels in one whole year.
恒星之间的距离太远了，我们用光年来量—— 也就是光在整整一年里走过的距离。
This is our own star, photographed in ultraviolet light.
这是我们自己的恒星，用紫外线拍摄的。
The Sun is a star of medium size, made mostly of hydrogen and helium.
太阳是一颗中等大小的恒星， 主要由氢和氦组成。
That bright loop at the edge is hot gas arching above the surface.
边缘那道明亮的环，是拱到表面之上的高温气体。
It looks calm from Earth, but the temperature in its core is about fifteen million degrees.
从地球上看它很平静，但它核心的温度大约有一千五百万度。
A star is powered by nuclear reactions in its core.
恒星的能量来自它核心里的核反应。
In a stable star, that reaction is nuclear fusion: hydrogen nuclei join together to make helium, and every join releases a huge amount of energy.
在一颗稳定的恒星里， 这个反应是核聚变：氢原子核结合起来生成氦，每一次结合都释放出巨大的能量。
That energy leaves the Sun as radiation spread across the electromagnetic spectrum, and most of it comes out as infrared, visible light and ultraviolet.
这些能量以辐射的形式离开太阳，分布在整个电磁波谱上， 其中大部分以红外线、可见光和紫外线的形式射出。
Stars are not spread out evenly.
恒星并不是均匀分布的。
They gather into galaxies.
它们聚集成星系。
A galaxy is a group of many billions of stars, all held together by gravity, and this one is a spiral.
星系是由数十亿甚至更多颗恒星组成的一群，全靠引力维系在一起，这一个是旋涡星系。
Our Sun is one ordinary star in the galaxy we call the Milky Way, which is about one hundred thousand light-years across — that distance is its diameter.
我们的太阳只是我们称为银河系的那个星系里一颗普通的恒星，银河系横跨约十万光年——这个距离就是它的直径。
Every other star in it is very much further away from us than the Sun is.
银河系里其他每一颗恒星，都比太阳离我们远得多。
Drawn to the same scale, the next nearest star sits off this frame.
按同样的比例画，下一颗最近的恒星会画到画面外面。
For the distances between stars, kilometres are useless.
要量恒星之间的距离，千米就没用了。
We use the light-year.
我们用光年。
A light-year is a distance, not a time: it is how far light travels in one year, about nine and a half million billion metres.
光年是一个距离，不是一段时间：它是光在一年里走过的距离， 大约是九点五乘以十的十五次方米。
Sunlight reaches us in eight light-minutes.
太阳光只要八个光分钟就到我们这里。
The next nearest star is over four light-years away.
离我们最近的另一颗恒星在四光年以外。
And the Milky Way is about one hundred thousand light-years across.
而银河系的直径大约是十万光年。
It is one galaxy among many billions, and all of them together are the Universe.
它只是数十亿个星系中的一个，而所有星系合在一起，就是宇宙。
Stars are born, they live, and they die.
恒星有生、有活、有死。
A star begins in a nebula — a cloud of gas and dust — pulled together by gravity until fusion ignites.
一颗恒星始于星云——一团气体和尘埃—— 在引力作用下聚拢，直到聚变被点燃。
For most of its life it shines steadily as a main-sequence star, like our Sun.
它一生的大部分时间， 像我们的太阳一样稳定地发光，是一颗主序星。
When its fuel runs low, it swells into a red giant.
当燃料快用完时，它膨胀成红巨星。
A star like the Sun then gently sheds its outer layers and shrinks into a white dwarf.
像太阳这样的恒星，接着会缓缓抛掉外层，收缩成一颗白矮星。
But a much heavier star explodes as a supernova, leaving behind a neutron star, or a black hole.
但一颗质量大得多的恒星，会以超新星的形式爆炸，留下一颗中子星，或者一个黑洞。
Now a star's life, one stage at a time.
现在来看恒星的一生，一个阶段一个阶段地看。
A star begins in an interstellar cloud of gas and dust that contains hydrogen.
恒星始于一团含有氢的星际云，里面有气体和尘埃。
Gravity pulls the cloud inwards, and as it collapses it heats up.
引力把这团云往里拉，它在坍缩的过程中升温。
That hot, collapsing cloud is a protostar.
这团又热又在坍缩的云就是原恒星。
It becomes a stable star when the inward pull of gravity is exactly balanced by an outward push caused by the very high temperature in its centre.
当引力向内的拉力，正好被中心极高温度造成的向外的推力平衡时，它就成为一颗稳定的恒星。
And it stays stable until it runs out of hydrogen fuel.
而它会一直稳定下去，直到把氢燃料用完。
What happens next depends on mass.
接下来会发生什么，取决于质量。
Take a medium star, like our Sun.
先看一颗中等质量的恒星，比如我们的太阳。
As the hydrogen runs low it swells up into a red giant, far bigger than it is today.
当氢快用完时，它膨胀成一颗红巨星，比今天大得多。
Then it gently throws off its outer layers, and that glowing shell of gas is called a planetary nebula.
然后它缓缓抛掉外层，那圈发光的气体壳叫做行星状星云。
Left behind in the middle is the small, hot core: a white dwarf, which just slowly cools.
留在中间的是又小又热的核心：一颗白矮星，它只会慢慢地冷却下去。
The Sun will end quietly, with no explosion.
太阳会安静地走完这一生，不会爆炸。
Now a much heavier star.
再看一颗质量大得多的恒星。
It swells into a red supergiant, and then it dies violently: it explodes as a supernova.
它膨胀成一颗红超巨星，然后剧烈地死去： 它以超新星的形式爆炸。
The blast spreads out a nebula of hydrogen and new, heavier elements.
这次爆发把一团含有氢和新生成的较重元素的星云抛散出去。
What is left in the middle is squeezed into a neutron star, or, if the star was heavy enough, a black hole.
留在中间的物质被压成一颗中子星，或者，如果这颗恒星足够重，就成为一个黑洞。
That scattered nebula can form new stars and planets later, so our own Solar System came from earlier stars.
而那团散开的星云以后还能形成新的恒星和行星， 所以我们自己的太阳系，来自更早的恒星。
Here is one of the biggest ideas in all of science.
这是整个科学中最重大的思想之一。
When we look at distant galaxies, their light is stretched towards the red end of the spectrum — a redshift.
当我们观察遥远的星系时， 它们的光被拉长、偏向光谱的红端——这就是红移。
The further away a galaxy is, the greater its redshift, and the faster it is racing away from us.
星系离得越远，红移就越大， 逃离我们的速度也越快。
The whole Universe is expanding.
整个宇宙都在膨胀。
Run that expansion backwards, and everything began about fourteen billion years ago, in a single event: the Big Bang.
把这种膨胀倒过来推， 一切都始于大约一百四十亿年前的一个事件：宇宙大爆炸。
Let's look at redshift more closely.
让我们把红移看得更仔细一些。
When a star or galaxy is receding — moving away from us — the light we receive has a longer wavelength than it should, and a longer wavelength means the light is shifted towards the red end of the spectrum.
当一颗恒星或一个星系在退行， 也就是正在远离我们时，我们收到的光的波长会比原本更长， 而更长的波长意味着光偏向了光谱的红端。
We can see it in the dark lines of a galaxy's spectrum: the pattern is the same, but the whole pattern has slid towards the red.
我们能在星系光谱的暗线上看出来：这些线的图案还是一样的， 但整个图案都朝红端滑动了。
And the further away the galaxy, the bigger the shift.
而星系离得越远，这个偏移就越大。
There is a second piece of evidence, and this picture is it.
还有第二个证据，就是这张图。
Faint microwave radiation reaches us from every single direction in space.
有一种微弱的微波辐射，从空间的每一个方向传到我们这里。
This is the cosmic microwave background radiation.
这就是宇宙微波背景辐射。
It started out as high-energy radiation soon after the Big Bang, and the expanding Universe has stretched it out into microwaves.
它一开始是大爆炸之后不久产生的高能辐射， 而不断膨胀的宇宙把它拉长，变成了微波。
To measure the expansion we need two numbers for a distant galaxy.
要测量这种膨胀，我们需要一个遥远星系的两个数据。
Its speed away from us comes from the redshift of its starlight.
它远离我们的速度，来自它星光的红移。
Its distance comes from how bright a supernova looks inside it.
它的距离，来自星系里一颗超新星看起来有多亮。
Big telescopes like this one, firing a laser to sharpen the image, collect both.
像这样的大型望远镜，用激光让图像更清晰，就能同时得到这两个数据。
The Hubble constant links those two numbers.
哈勃常数把这两个数据联系起来。
It is the speed of a galaxy divided by its distance.
它等于一个星系的速度除以它的距离。
Its value today is about two point two times ten to the minus eighteen, per second.
它今天的数值大约是每秒二点二乘以十的负十八次方。
Now turn that division upside down.
现在把这个除法反过来。
Distance divided by speed is one over the Hubble constant, and distance divided by speed is a time: how long the galaxies have been travelling.
距离除以速度，就是一除以哈勃常数， 而距离除以速度是一个时间：星系已经走了多久。
So let's estimate the age of the Universe.
那我们来估算一下宇宙的年龄。
Take one over the Hubble constant.
取一除以哈勃常数。
That is one divided by two point two times ten to the minus eighteen.
也就是一除以二点二乘以十的负十八次方。
Doing the division gives four point five times ten to the seventeen seconds.
算出来是四点五乘以十的十七次方秒。
Seconds are hard to picture, so change them into years: it comes out at roughly fourteen billion years.
秒很难想象，所以把它换成年： 结果大约是一百四十亿年。
Every galaxy seems to have set out from the same point at the same time, and that is more support for the Big Bang.
看起来每一个星系都是在同一时间从同一点出发的， 这又是支持大爆炸的一个证据。
Three marks to lock in.
锁住三个分。
First, day and night come from the Earth spinning on its axis, while a year comes from it orbiting the Sun — never mix them up.
第一，白天和黑夜来自地球绕地轴自转，而一年来自它绕太阳公转—— 千万别搞混。
Second, orbital speed is the circumference, two pi times the radius, divided by the period — not the other way round.
第二，轨道速度是圆周长，也就是二派乘以半径，再除以周期——不是反过来。
Third, a bigger redshift means a galaxy is further away and moving away faster — the evidence that the Universe is expanding.
第三，红移越大，说明星系离得越远、逃离得越快——这正是宇宙在膨胀的证据。
Master these, and space physics is yours.
把这些掌握好，空间物理就是你的了。
Three more marks to lock in.
再锁住三个分。
A light-year is a distance, not a time — it is how far light travels in one year, so never use it as a length of time.
光年是一个距离，不是一段时间——它是光在一年里走过的距离， 所以千万不要把它当成一段时间。
Next, a planet closer to the Sun feels stronger gravity, so it orbits faster and its year is shorter.
第二，离太阳更近的行星受到的引力更强， 所以它公转得更快，一年也更短。
And in a stretched, elliptical orbit a body speeds up as it nears the Sun, because gravitational potential energy turns into kinetic energy.
第三，在被拉长的椭圆轨道上， 物体靠近太阳时会加速，因为重力势能转化成了动能。
And three to finish with.
最后再来三个。
A star is stable when the inward pull of gravity is balanced by the outward push from the energy released by fusion in its core — say both halves of that sentence.
当引力向内的拉力，被核心里聚变释放的能量造成的向外的推力平衡时， 恒星就是稳定的——这句话的两半都要说出来。
What the star becomes when its fuel runs out depends on its mass, and nothing else.
恒星在燃料用完之后变成什么，只取决于它的质量。
And in the orbital speed equation, put the radius in metres and the period in seconds before you divide.
还有，在轨道速度公式里，先把半径换成米、把周期换成秒，再做除法。
Get those right, and space physics is straightforward marks.
把这些做对，空间物理就是稳稳的分数。