Science Helpdesk


ESO 1. Science

Topic 1.  Reading: Astronomers Await a Nova
The Star of Bethlehem

At this time of year it seems almost traditional for stargazers to ponder the age-old question of the origin of the Star of Bethlehem. The Star's appearance some 2,000 years ago is quite possibly one of the best-known celestial events in all of recorded history.

The topic has universal fascination, and is why Christmas Star shows still play to packed planetarium houses.

Perhaps the simplest answer that can be offered is that the Star might have been a nova: a new star suddenly blazing forth where no star had previously been seen. While for the most part such objects are really dying stars having a final fling of glory before descending the long road to ultimate extinction, there are some stars that go through such contortions more than once.

One such star is long overdue to pop and could do so at anytime.


The star in question is T Pyxidis, in the constellation of Pyxis, the Mariner's Compass. T Pyxidis is about 6,000 light years away and belongs to a small and seemingly exclusive group of cataclysmic variable stars called recurrent novae (NR). Astronomers have been patiently waiting for T Pyxidis's next outburst for more than 20 years.

Normally this star shines at magnitude 14: that's about a thousand times dimmer than the faintest star that can be perceived by most human eyes on a dark, clear night. But on five occasions, in 1890, 1902, 1920, 1944 and 1967, this star brightened dramatically to magnitudes between 6.5 and 7 (a 1,000-fold increase in brightness in the most extreme case) making T Pyxidis just bright enough to be glimpsed without any optical aid. These eruptions came at an average of just over 19 years apart, and the longest stretch of time between them was 24 years.

But this month marks 40 years since the last outburst.

It was back on Dec. 7, 1966 that the most recent eruption was first noticed by New Zealand amateur astronomer, Albert Jones. The star had more than doubled in brightness to magnitude 12.9. Just two nights later it was almost four magnitudes brighter and after a month it was glowing at magnitude 6.3 before slowly fading back to normal.

Nobody knows exactly why T Pyxidis has remained quiet for so long, but the general consensus is that it may have accumulated an extra-thick coating of nuclear fuel on its surface over these past 20 years, which would make it appear extra bright when it finally blows its next surge of gaseous debris out into space.

Who knows? That night could be tonight!

Key vocabulary
  • Stargazer.
  • Outburst.
Analyse the text
  • What are astronomers expecting to happen and to what?
  • What is a nova?
Topic 8.  Do it yourself: Making a Plant Collection
[Adapted from The University of Arizona]
What is it?

When scientists preserve a specimen of a plant (or part of a plant) they usually flatten it, dry it, and mount it on special paper. Preserved in this way the plant specimen can be stored for many years without falling apart.

Before you start

Before collecting plants in the wild, you should understand the legal issues of the ownership of the land and its resources, and the ethical issues of possible damage to wild plant populations and to endangered species. It is legal to collect plants only with the permission of the owner of the property on which they are found.

What to collect

Picking a few leaves or flowers usually does not give a representative picture of a plant. Pieces of specimen plant material need to be large enough to show the characteristics of normal growth and development. Taking a branch, stem or even the entire plant may be required to get a good specimen. If the plant is small, take the whole thing, roots and all, or even several of them. If large, get a branch about 25 cm long, with leaves, flowers, and fruits, if possible. A "sterile" specimen (one with leaves only) may be impossible to identify. Even an old empty seed capsule can be helpful if that's all you can find.

Information needed

The date the plant was collected and the location as exactly as possible. Record anything that the specimen won't show, for example, the size of the plant, flower colour, whether the plant is woody or not, etc. Note what kind of a place the plant was found, e.g., in gravel at stream edge, in shade under live oaks, in a sidewalk crack…

How to press a plant

Place the specimens between newspaper sheets and write the name of the species alongside. Alternatively, you can separate the specimens with corrugated cardboard (for air circulation) and blotter paper or paper towels to absorb moisture. Arrange the plant so that all parts show (for example, don't put the flowers between layers of leaves). Place the stack between boards and strap them tightly or place a heavy weight on top (such as a pile of heavy books). Put the stack where there is good air circulation, but not too much heat: you don't want to cook them.

Examine the plants daily and change newspapers or blotters when they become too wet. Remove plants from the stack when they are dry. You can kill insects in dried plant specimens by freezing them for three or four days, and keep them pest-free in a tightly sealed plastic bag.

The herbarium

Once the plants are dry enough, stick each specimen in a cardboard folder. Label them writing their Latin (scientific) and English names, along with all the other data that you previously recorded (collection date and place, description of the specimen...).

ESO 2. Science

Topic 6.  Exercises: Balance the following chemical equations
  1. C4H10 + O2 → CO2 + H2O
  2. H2O → O2 + H2
  3. H2S + O2 → SO2 + H2O
  4. HCl + Al → AlCl3 + H2
  5. NO → N2 + O2
  6. N2 + H2 → NH3
  7. Fe + O2 → Fe2O3
  8. KClO3 → KCl + O2
  9. Mg + Fe2O3 → MgO + Fe
  10. Na + Ag2O → Ag + Na2O
  11. CO + O2 → CO2
  12. C2H5OH + O2 → H2O + CO2
  13. SO + O2 → S2O3
  14. Mg + Ag2O → Ag + MgO
  15. Na + Fe2O3 → Na2O + Fe
  16. H2SO4 + Zn → ZnSO4 + H2
  17. Zn + HCl → ZnCl2 + H2
  18. CaCO3 → CaO + CO2
  19. N2O5 + H2O → HNO3
  20. HNO3 + NaOH → NaNO3 + H2O
  21. H2SO4 + KOH → K2SO4 + H2O
  22. H2CO3 + NaOH → Na2CO3 + H2O
  23. H2S + CaCO3 → CaS + H2O + CO2
  24. NaCl + AgNO3 → NaNO3 + AgCl
  25. HCl + CaCO3 → CaCl2 + H2O + CO2

ESO 3. Biology and Geology


ESO 4. Biology and Geology

Topic 4.  Questions: Earth's Timeline
  1. Why did the first oceans and continents form at about the same time?
  2. All or almost all rocks known from the Archean eon are metamorphic. Why?
  3. According to the biochemical fossils of Isua (Greenland), it is said that Life appeared on Earth in the very first moment that it was possible. Why?
  4. The oxygen pollution was necessary for Life to colonize the continents. Why?
  5. Which eukaryotic organelles are thought to have evolved from bacteria engulfed by the primitive eukaryotic ancestor?
  6. Why did sexual reproduction appear so late in the History of Life on Earth?
  7. Why does sexual reproduction increase the rate of evolutionary change?
  8. Considering the huge amount of fossils that these organisms left behind, the Paleozoic is known as the era of the ...
  9. From which geological time comes most of the coal that we consume nowadays?
  10. Considering the way these animals dominated the land ecosystems, the Mesozoic is known as the era of the ...
  11. The rise of angiosperms in the late Mesozoic must have boosted the evolution of ...
  12. How can a meteorite cause such a massive extinction as that of the end of the Mesozoic?
  13. Some scientists say that dinosaurs have not died off, and that they are still among us. Why?
  14. Mammals had existed throughout all the Mesozoic, but why didn't they flourish until the Cenozoic?
Topic 4.  Questions: Fossils
  1. Why are fossils unlikely to be found in granite?
  2. Why are fossils unlikely to be found in gneiss?
  3. Why can fossils be found in sandstone?
  4. Why can fossils be found in slate?
  5. An unknown fossil is found in a rock with symmetrical ripple marks, such as those left in the sand by the sea waves. What sort of information does this provide about the life habits of the organism that left the fossil?
  6. Can you tell any other way by which a sedimentary rock can provide useful information about the fossils found in them?
  7. What is the difference between an internal mould and a cast?
  8. What sort of fossils can be expected from a poppy? And from an oak tree? And from a grasshopper?
  9. What kind of trace fossils can a tiger leave? And a trilobite?
  10. Why do you have a greater proportion of 12C than the atmosphere?
Topic 4.  Reading: How the Discovery of Geologic Time Changed our View of the World

Imagine trying to understand history without any dates. You know, for example, that the First World War came before the Second World War, but how long before? Was it tens, hundreds or even thousands of years before? In certain situations, before radiometric dating, there was no way of knowing.

By the end of the 19th century, many geologists still believed the age of the Earth to be a few thousand years old, as indicated by the Bible, while others considered it to be around 100 million years old, in line with calculations made by Lord Kelvin, the most prestigious physicist of his day.

Dr Cherry Lewis, University of Bristol, UK, said: "The age of the Earth was hugely important for people like Darwin who needed enormous amounts of time in which evolution could occur. As Thomas Huxley, Darwin's chief advocate said: 'Biology takes its time from Geology'."

In 1898 Marie Curie discovered the phenomenon of radioactivity and by 1904 Ernest Rutherford, a physicist working in Britain, realised that the process of radioactive decay could be harnessed to date rocks.

It was against this background of dramatic and exciting scientific discoveries that a young Arthur Holmes (1890-1964) completed his schooling and won a scholarship to study physics at the Royal College of Science in London. There he developed the technique of dating rocks using the uranium-lead method and from the age of his oldest rock discovered that the Earth was at least 1.6 billion years old (1,600 million).

But geologists were not as happy with the new results as, perhaps, they should have been. As Holmes, writing in Nature in 1913, put it: "the geologist who ten years ago was embarrassed by the shortness of time allowed to him for the evolution of the Earth's crust, is still more embarrassed with the superabundance with which he is now confronted." It continued to be hotly debated for decades.

Cherry Lewis commented, "In the 1920s, as the age of the Earth crept up towards 3 billion years, this took it beyond the age of the Universe, then calculated to be only 1.8 billion years old. It was not until the 1950s that the age of the Universe was finally revised and put safely beyond the age of the Earth, which had at last reached its true age of 4.56 billion years. Physicists suddenly gained a new respect for geologists!"

Answer the following questions:
  • How do the radiometric dating techniques work?
  • Why were the radiometric dating techniques so important for Biology after Darwin's time?
Topic 4.  Reading: Brief History of Paleoanthropology
In the beginning...

Today, it's a widely accepted fact that humans originated in Africa. But less than a century ago, anthropologists assumed that Eurasia was the birthplace of humanity. And scientists held onto that mistaken belief until one man took a stand that rewrote history.

In 1923, Raymond Dart arrived at the University of the Witwatersrand in Johannesburg to take a post as the head of its anatomy department. The 30-year-old Australian physician, an expert in neuroanatomy, was disappointed to learn that the university did not own a reference collection of bones and fossils. He set out to amass one, offering his students a prize for the most interesting bones they could find. His lone female student, Josephine Salmons, soon presented him with a South African fossil that would lead to the discovery of a lifetime. The fossil, a baboon cranium, sparked Dart's interest, since only two primate fossils had been found in sub-Saharan Africa until then. Salmons had found the fossil at the home of the director of the Northern Lime Company at Taung, a mining site in South Africa. Dart asked the manager of the site to alert him to any fossils that his miners unearthed in the future.

The Child of Taung

One Saturday in 1924, two boxes of rocks from Taung were deposited at Dart's door. In the second box, he came across an exciting find: an endocast, the fossilised imprint of an animal brain. To his astonishment, the endocast showed a brain larger than that of a chimpanzee but smaller than those of known human ancestors.

Digging through the box, Dart located the matching limestone rock that he knew might contain the face to match the brain. His thoughts turned to Charles Darwin, who, in his 1871 book The Descent of Man, predicted that humans' earliest apelike ancestors would be discovered in Africa because our closest ape cousins - chimpanzees and gorillas - lived there. Darwin's prediction had been generally discredited. After all, only two ancestral human fossils had ever been found in Africa and both were relatively recent, closer to modern humans than to apes. Dart wondered whether he had stumbled upon proof of Darwin's controversial theory.

For the next 73 days, Dart scraped away at the matrix, the limestone surrounding the fossilised face. Finding his hammer and chisel too clumsy, Dart turned to his wife's knitting needles, which he had sharpened to a point. His efforts were rewarded when the rock finally parted to reveal the face of an apelike child with a full set of baby teeth and molars beginning to appear.

Despite the primitive face, the child's skull, teeth and jaw clearly resembled those of humans, and the position of the opening at the back of the skull, where the spinal cord meets the brain, indicated that it walked upright, known as bipedalism. Here, Dart realized, was an early human ancestor. In a 1925 article in the journal Nature, Dark introduced the new species, which he named Australopithecus africanus, meaning "southern ape from Africa". The fossil became known as the Taung child.


Dart's discovery set off a firestorm. The leading European scientists, who preferred to believe that humans originated closer to home, were skeptical of his find. An abundance of physical evidence, including fossils and cave paintings, seemed to point to Eurasia as the cradle of humankind, and a British fossil known as Piltdown man was the most convincing evidence that Dart was incorrect. Charles Dawson, an English soldier and amateur antiquarian, discovered the bones in a gravel pit in Piltdown Common in Sussex between 1910 and 1912. Not only was Piltdown man conveniently European, but it looked like what scientists expected human ancestors to look like. It had a simian jaw and teeth but a modern-human-size brain. At the time, scientists assumed that our large brain evolved before other human traits. Piltdown man was, therefore, considered irreconcilable with the Taung child, with its humanlike jaw and teeth but small brain.

In Europe, scientists and the press widely dismissed the Taung child and ridiculed Dart. Most paleontologists believed the fossil was of a young ape, most likely a chimpanzee. Dart traveled to England in 1931 to show the fossil at scientific conferences, but he did not gain many supporters. He did not touch the fossil again for years.

Dart was not the first researcher whose fossil was unfairly rejected by European scientists. In 1893, Dutch physician Eugene Dubois announced the discovery of a set of well-preserved fossils from the Indonesian island of Java. Dubois felt that the three bones - a skullcap, molar and femur - belonged to a human ancestor that walked upright. He called the species Pithecanthropus erectus, or upright ape-man, and the fossil became known as Java man.

When he returned to Europe, Dubois was surprised to be met with resistance. Like the Taung child, Java man's skullcap indicated a relatively small brain. The human brain, scientists said, must have reached modern proportions before the creature could have been capable of walking upright. Dubois's colleagues immediately dismissed the fossil as a large gibbon or a deformed modern human.

In 1940, Dubois died without receiving the credit he was due. Java man has since been reclassified as Homo erectus, the enormously successful species that thrived for 1.5 million years starting around 1.7 million years ago and that may have directly preceded Homo sapiens, or modern humans. As the first H. erectus specimen ever found, the 900,000-year-old Java man is among the most important human-lineage, or hominan, fossils in the world.

New African discoveries

Fortunately for Raymond Dart, thanks to some important allies, recognition came within his lifetime. Although European scientists rejected his findings, his article in Nature inspired a colleague, the Scottish scientist Robert Broom, to prove that Dart's fossil was indeed an early human and that our earliest ancestors hailed from Africa. At the age of 70, Broom, a respected paleontologist and curator of vertebrate fossils at the Transvaal Museum in Pretoria, set out to find an adult Australopithecus in Africa - something to silence Dart's critics once and for all.

Broom found such evidence in 1936, when he began acquiring fossils from caves at Sterkfontein, a site just south of Johannesburg. His early finds included the skull of an adult australopithecine. After World War II, Broom found the limb bones of a different hominan species, Paranthropus robustus, at a nearby site. With the two fossils in hand, Broom was able to confirm that these early ancestors walked upright.

These finds infected Dart's students with enthusiasm, and one of them convinced him to go fossil-hunting in the 1940s. In the caves at Makapansgat, 150 miles north of Sterkfontein, Dart and his colleagues found several australopithecines. But Dart wasn't truly vindicated until 1947, when Broom found the fantastically well-preserved skull of an adult female A. africanus at Sterkfontein. The evidence could no longer be denied: The Taung child was a hominan, not an ape. That same year, the British Association for the Advancement of Science passed a resolution stating that Broom's discoveries provided "a vindication of the general view put forward by Professor Raymond Dart in his report of the first Australopithecus skull found in 1924."

A theory takes hold

By 1953, Piltdown man had become a clear anomaly in the growing hominan fossil record. That year, scientists at Oxford and the British Museum revealed that the English fossil had, in fact, been a hoax. The culprit - whose identity remains a mystery - pieced together the fossil from a 600-year-old human cranium, an orangutan jaw and teeth, and perhaps a chimp tooth. The bones had been chemically stained and the teeth worn down to mimic human usage. With Piltdown man out of the way, the African-origin theory gained widespread acceptance.

The final confirmation came in 1959, when British archaeologist Mary Leakey made an unexpected discovery in East Africa's Rift Valley. Out walking her dogs one morning in Tanzania's Olduvai Gorge, Leakey noticed a bone sticking out of the sand. When she brushed the dirt off, she was staring into the dark eye sockets of a near-complete skull of a previously unknown species, one that her husband, archaeologist Louis Leakey, later named Zinjanthropus boisei. The fossil, with its huge cheekbones and sharply crested skull, was an amazing find. And like the Taung child, it had a brain smaller than that of modern humans but larger than modern apes.

In 1961, geologists at the University of California at Berkeley used a new technique called potassium-argon dating to accurately place the species, whose scientific name had been changed to Paranthropus boisei, in history. According to this groundbreaking method, which involves dating the rocks surrounding the specimen, the skull was 1.75 million years old - four times as old as previously thought and 750,000 years older than the accepted time frame for all of human history.

Nutcracker man, as the skull was known, was the conclusive fossil evidence that the earliest human ancestors lived in Africa, despite the fact that they didn't look like what scientists expected. Researchers streamed to East Africa in a fossil rush aimed at finding our earliest ancestors. Over the next few decades, a series of spectacular discoveries in East Africa pushed human origins deeper into the past.

In 1974, researchers in the Afar region of Ethiopia discovered "Lucy", an astonishingly complete three-million-year-old A. africanus skeleton. Lucy could clearly walk upright and was considered our oldest ancestor for a time. More recently, fossil finds have pushed the date of our earliest ancestor back further, to between five million and six million years ago, and in 2002, French paleontologist Michel Brunet announced the discovery of the oldest species yet, the six-to-seven-million-year-old Sahelanthropus tchadensis. Brunet found Toumaï, as the oldest hominan fossil ever found has come to be known, in the Djurab Desert of northern Chad.

Beyond fossils

In the 1960s, when potassium-argon dating was developed, paleontology began to go high-tech. For the first time, fossils older than 50,000 years could be dated. Genetic technology also came into play that decade, and today it is revolutionizing the study of human origins.

Vincent Sarich and Allan Wilson of the University of California at Berkeley were pioneers in the application of genetics to paleoanthropology in the 1960s. Their comparisons of chimpanzee and human DNA showed that the chimp lineage split from our common ancestor five million years ago. At the time, the findings caused an uproar. Anthropologists, who believed that humans and chimps diverged 15 million years ago, rejected the theory.

Recent genetic studies, however, support Sarich and Wilson's early work. The complete human genome was published in 2004, and the chimp genome followed in 2005. That year, scientists at Arizona State University and Pennsylvania State University compared modern human mitochondrial, or maternal, DNA with chimpanzee, macaque and mouse DNA to determine the point at which each lineage diverged from our common ancestor. "Though we will never know the exact date of the split, we can estimate that date using differences in their DNA", explains Blair Hedges, an evolutionary biologist at Penn State. These differences, or mutations, are assumed to occur at a constant rate, which can be used to estimate how much time has passed since lineages diverged. This method, called the molecular clock, indicated that the human and chimp lineages split five million to seven million years ago, although more fossil-based research is needed to confirm that idea.

Key vocabulary:
  • Amass.
  • Endocast.
  • Unearth.
  • Hominan.
  • Foramen magnum.
  • Bipedalism / Bipedality.
Answer the following questions:
  • Which was the presumed cradle of Mankind by the beginning of the XX century?
  • What discovery lead finally to the dismissal of that theory?
  • What species does the Child of Taung belonged to?
  • What is an endocast?
  • What type of human evolution was suggested by the Piltdown Man?
  • And what type of human evolution was suggested by the Child of Taung?
  • What do the huge cheekbones and sharply crested skull of Paranthropus boisei tell about its life habits?
  • How does the genetic clock technique work?
  • What does the genetic clock technique say about the split date between human and chimpanzee lineages?
  • Which is the oldest hominan known to date? How old is it?
Topic 5.  Questions: Plant Transpiration
[Source lesson]
  1. Which are the two physical processes that allow the loss of water in the leaves?
  2. "If a plant wants to obtain water, it has to lose water". Is this assertion true? Why?
  3. Why is it good for a plant to lose water by transpiration?
  4. Why is it bad for a plant to lose water by transpiration?
  5. Why is transpiration important to keep a plant upright?
  6. How does transpiration make the replication of DNA possible?
  7. Where is the boundary layer thicker: around the leaves of a cactus or around the leaves of a poplar? Why?
  8. How does the rate of transpiration change when you take a plant from inside a house to a garden? Why?
  9. How does the rate of transpiration change when you start boiling water in the same room where you have a plant? Why?
Topic 5.  Questions: Plant Reproduction
  1. There are two ways in which a plant can be hermaphrodite. Which ones?
  2. What is the minimum composition of a male flower?
  3. The anthers of grasses are very big and dangle out of the flower. Do you expect these flowers to have a colourful big corolla? Why?
  4. Which one of the three main parts of a pistil is the least necessary? Why?
  5. How many nuclei does each pollen grain have when it is fully developed?
  6. What is the mission of the tube nucleus of a pollen grain?
  7. There is one egg-cell inside each _______________. One ovary can contain one or several _______________.
  8. When fertilized, the ovules develop into _______________ and the ovaries that contain them develop into _______________.
  9. What is the micropyle?
  10. Why is plant fertilization called "double fertilization"?
  11. What are the main components of a seed? What is the function of each?
  12. How can a fleshy fruit help the dispersal of its seeds?