0625 · 6.2Stars and the Universe

Stellar energy and stability

0:00Sunlight and the faint light from distant galaxies carry information about objects we cannot visit. That information reveals how stars release energy and how the Universe changes. Even the wavelengths of light help us reconstruct the past. By the end, you will understand stellar life cycles and the evidence supporting the Big Bang Theory. Three familiar ideas will help us explain these enormous objects. Gravity attracts masses towards one another, and hotter gases produce greater pressure.

0:27Electromagnetic radiation carries energy through space without needing a material medium. Its wavelength is the distance between neighbouring crests. Each range of wavelengths belongs to a particular region of the electromagnetic spectrum. The Sun is a star of medium size, consisting mostly of hydrogen and helium. It radiates most of its energy in the infrared, visible light and ultraviolet regions of the electromagnetic spectrum. Visible light is only the part our eyes detect.

0:55The other regions also carry energy, even though we cannot see them directly. For Supplement, or Extended, you must understand where a star gets its energy. Stars are powered by nuclear reactions that release energy. In stable stars, these reactions involve the fusion of hydrogen into helium. Fusion means that light atomic nuclei join to form a heavier nucleus, and this process releases energy. The Sun is therefore not burning like a candle or a coal fire.

1:21Ordinary burning is a chemical reaction, whereas fusion changes atomic nuclei. Conditions in the centre of a star allow hydrogen nuclei to fuse. The energy released eventually leaves the star as electromagnetic radiation, including the sunshine that reaches Earth. Gravity continually pulls the material of a star inward. Meanwhile, the high temperature in its centre produces pressure that pushes outward. In a stable star, these inward and outward effects balance.

1:48Stable does not mean inactive, because fusion continues while energy escapes into space. Think of this balance as a competition between gravity and pressure. Without sufficient outward pressure, gravity would make the star contract. Without enough gravitational attraction, the material would spread outward. The energy supplied by fusion helps maintain the high temperature needed for this balance, so the star can remain stable.

Light-years and distant starlight

2:12The Sun belongs to the galaxy known as the Milky Way. A galaxy is an enormous collection containing many billions of stars. The other stars in our galaxy are much further from Earth than the Sun is. Their faint appearance does not necessarily mean they release little energy, because distance greatly affects observed brightness. Astronomical distances are so large that metres often become inconvenient. We therefore use a unit called the light-year.

2:36One light-year is the distance travelled in the vacuum of space by light in one year. Despite its name, it measures distance like the metre, not time. For Extended, you must know that one light-year equals 9.5 × 10¹⁵ metres. We can understand this value using distance equals speed multiplied by time. Light travels at approximately three hundred million metres per second in a vacuum. Multiplying that speed by the seconds in a year gives an enormous distance.

3:04One year contains approximately 3.15 × 10⁷ seconds. Multiply this by 3.0 × 10⁸ metres per second. The result is approximately 9.5 × 10¹⁵ metres. That calculation explains the distance represented by one light-year. Suppose a question gives a star’s distance as four light-years and asks for this value in metres. Each light-year represents the same distance, so multiply four by 9.5 × 10¹⁵. This gives 3.8 × 10¹⁶ metres.

3:40Light from that star takes four years to reach us. Looking at a distant star therefore means receiving light that left it in the past. We do not see its condition at this exact moment. This delay follows directly from the time light needs to cross space. The light-year combines a useful distance unit with an intuitive indication of how old the arriving light is.

Stellar life cycles

4:00The life cycle of a star is Supplement content for Extended students. A star forms from an interstellar cloud of gas and dust containing hydrogen. Interstellar simply means between stars. Gravitational attraction draws the material together, causing parts of the cloud to collapse and become increasingly concentrated. This collapsing cloud is called a protostar, and it is not yet a stable star. As gravity pulls its material inward, the temperature increases.

4:27The collapse transfers gravitational potential energy into the thermal store of the gas. Eventually, the centre becomes hot enough for nuclear fusion to provide a sustained energy supply. A protostar becomes a stable star when inward gravitational attraction balances the outward force due to its high central temperature. Hydrogen fusion supplies the energy needed to maintain that temperature. The star then spends a long period in this stable condition. Its later development depends strongly on how massive it is.

4:56All stars eventually run out of hydrogen as fuel for their nuclear reactions. The important turning point comes when most hydrogen in the centre has been converted to helium. This does not mean every hydrogen atom throughout the star has disappeared. Instead, the central fuel supply changes, and the conditions that maintained the stable stage are disrupted. Changes in the centre cause the outer regions to expand. Most stars become red giants, while more massive ones expand into red supergiants.

5:24Their expanded surfaces are cooler than before, giving them a redder appearance. A cooler surface does not mean the whole star is cold, because the centre remains extremely hot. For a less massive star, the red giant eventually loses its outer layers. These spread into space, forming a planetary nebula. A white dwarf remains at the centre. Despite the name, a planetary nebula is a cloud of material from a dying star, not a collection of planets.

5:50The white dwarf is the small, hot remnant of the star’s central region. It no longer maintains itself through hydrogen fusion like a stable star. Instead, it gradually radiates away its remaining thermal energy. For this course, the essential sequence ends with the planetary nebula and its central white dwarf. A more massive star follows a different route after becoming a red supergiant. It explodes as a supernova, ejecting material into space.

6:15This forms a nebula containing hydrogen and new heavier elements. At its centre, the explosion leaves behind either a neutron star or a black hole. A neutron star is an extremely dense stellar remnant. A black hole has such strong gravitational attraction that light cannot escape from within its boundary. You do not need detailed models of either object here. The key distinction is that these remnants come from massive stars, whereas white dwarfs form from lower-mass ones.

6:43The material released by a supernova does not have to remain scattered forever. Gravity can draw parts of that nebula together to form new stars. Some surrounding material can form planets orbiting those stars. A supernova therefore supplies material for later generations of stars and planetary systems. Keep the two pathways separate rather than treating every stage as one long sequence. A less massive star becomes a red giant, then leaves a planetary nebula and white dwarf.

7:10A more massive star becomes a red supergiant, then undergoes a supernova. Its central remnant is a neutron star or black hole.

Redshift and Big Bang evidence

7:18The Milky Way is only one of many billions of galaxies making up the Universe. Its diameter is approximately one hundred thousand light-years. That means light would take approximately one hundred thousand years to cross that distance. Individual galaxies are enormous, but the Universe contains vast numbers of them. To investigate distant galaxies, astronomers study the light reaching Earth. Different elements produce recognisable patterns of lines in their spectra. Those patterns can be compared with light emitted on Earth.

7:47If the pattern appears at longer wavelengths, we know that the arriving radiation has undergone redshift. Redshift is an increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies. Receding means moving away from us. On a visible spectrum, longer wavelengths lie towards the red end. The term describes a wavelength change, not a requirement for the whole galaxy to look red. Spectra from distant galaxies appear redshifted compared with light emitted on Earth.

8:16This provides evidence that those galaxies are receding. Across large distances, this pattern indicates that the Universe is expanding. Space between widely separated galaxies increases, rather than every object simply travelling away from Earth through an unchanged background. An expanding pattern does not make Earth the centre of the Universe. Imagine dots drawn on a strip that stretches uniformly. From any dot, the others become more distant as the strip lengthens.

8:42This analogy describes changing separations, although real space has three dimensions rather than the single one shown here. If the Universe is expanding now, tracing its history backwards brings matter closer together. This supports the Big Bang Theory, in which the Universe began in a very hot, dense state. It was not an ordinary explosion into surrounding empty space. The central idea is that the Universe itself has expanded from that early condition.

9:08Extended students need a second important piece of evidence called cosmic microwave background radiation. Microwave radiation with a characteristic frequency range is observed throughout the space around us. It reaches us from all directions. This widespread background is different from radiation associated with one particular nearby star or galaxy. Cosmic microwave background radiation was produced shortly after the Universe formed. As the Universe expanded, the wavelengths of this radiation became longer.

9:37The radiation is now observed in the microwave region of the electromagnetic spectrum. Together with redshift, it provides evidence of a hot early Universe. The syllabus describes background microwaves of a specific frequency. Understand this as a characteristic microwave background, rather than every wave having exactly the same frequency. The essential explanation concerns its origin and stretching during expansion. Do not confuse it with microwaves recently emitted by modern stars or equipment on Earth.

Hubble constant and Universe age

10:05The remaining calculations are Supplement content for Extended students. The speed at which a galaxy moves away can be found from the wavelength change caused by redshift. A far galaxy’s distance can be determined using the brightness of a supernova within it. These provide two separate measurements needed to investigate expansion quantitatively. The supernova method uses a suitable explosion whose actual brightness can be estimated. Compare that with how bright it appears from Earth.

10:32A more distant source appears fainter because its light spreads over a greater area. Without an estimate of the explosion’s true output, the observed brightness alone could not reveal distance. For distant galaxies following the general expansion, greater distance corresponds to faster recession. The Hubble constant is defined as the ratio of recession speed to distance from Earth. In words, the Hubble constant equals speed divided by distance. Equivalently, recession speed equals the Hubble constant multiplied by distance.

11:02This constant describes the expansion rate, rather than the speed of one particular galaxy. When speed is in metres per second and distance is in metres, the length units cancel. The remaining unit is per second. For this syllabus, the current estimate is 2.2 × 10 to the power of minus eighteen per second. Suppose a galaxy recedes at 6.6 × 10⁶ metres per second. Its distance is 3.0 × 10²⁴ metres.

11:30To calculate the Hubble constant, divide its recession speed by its distance. These quantities already use compatible units, so no conversion is needed. Dividing six point six by three point zero gives two point two. Dividing 10⁶ by 10²⁴ gives ten to the power of minus eighteen. Therefore, the Hubble constant is 2.2 × 10 to the power of minus eighteen per second. The unit follows from cancelling metres between numerator and denominator.

11:58A common calculation mistake is combining kilometres per second with a distance in metres. That produces the wrong numerical value unless you convert the units first. Another mistake is reversing the division. Distance divided by speed gives a time, whereas the Hubble constant measures an expansion rate with the unit per second. Distance divided by speed represents a travel time. Applying this to a receding galaxy gives an estimate of how long expansion has been occurring.

12:25Since the Hubble constant equals speed divided by distance, its reciprocal equals distance divided by speed. Therefore, one divided by the Hubble constant estimates the age of the Universe. This estimate imagines running the expansion backwards using the present rate. The galaxies would approach one another until their separations became extremely small. In the syllabus’s simplified model, this supports all matter having been present at a single point. It is an estimate, because the expansion rate has not necessarily remained unchanged throughout cosmic history.

12:57To estimate the age, divide one by 2.2 × 10 to the power of minus eighteen per second. This gives approximately 4.5 × 10¹⁷ seconds. To convert into years, divide by 3.15 × 10⁷. The result is approximately fourteen billion years.

Exam answers and recap

13:16An exam might ask you to define redshift and explain how it supports the Big Bang Theory. Begin with the measured change, then connect it to motion and expansion. Redshift is an increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies. Redshift in light from distant galaxies shows that they are moving away. The Universe is therefore expanding. Tracing this expansion backwards suggests that matter was previously concentrated into a much smaller region.

13:44This supports the Big Bang Theory. Each sentence supplies a distinct point, rather than repeating that galaxies are distant or simply saying their light looks red. For a stellar life-cycle question, describe the sequence appropriate to the mass stated. Include gravitational collapse and heating when explaining how a protostar forms. Explain stability through balanced inward gravitational attraction and outward pressure. Do not say fusion pushes the star outward directly, because its energy maintains the temperature responsible for that pressure.

14:15The main ideas fit together through gravity, nuclear energy and electromagnetic radiation. Gravity gathers stellar material, while hydrogen fusion helps maintain a stable star. The initial mass determines whether the later stages leave a white dwarf, neutron star or black hole. Material from a supernova can become part of new stars and orbiting planets. Light-years measure distance, and redshift reveals increasing wavelengths in radiation from receding galaxies. Galactic redshift and cosmic microwave background radiation support an expanding Universe with a hot, dense beginning.

14:51The Hubble constant connects recession speed with distance. Its reciprocal estimates the time over which cosmic expansion has occurred.