Star Formation: Pressure and Temperature

Overview

Stars form when giant clouds of gas collapse under their own gravity. As the gas is compressed, its temperature rises — and this temperature rise is ultimately what ignites nuclear fusion. But why does compressing a gas heat it up? This experiment explores the relationship between pressure and temperature directly: a TI SensorTag is placed inside a sealed syringe or press-plunger container, and the gas is rapidly compressed. phyphox plots both pressure and temperature in real time, making the physical connection visible. This is a classroom analogy; the phyphox file does not model star formation directly.

Grades	7–10
Subject	Physics, Natural Sciences
Method	Student experiment
Duration	Variable
Language	German, English
Sensor	One TI SensorTag (pressure in hPa + temperature in °C, 1 sample/s)

Background

When a gas is compressed quickly (adiabatically — faster than heat can escape), the work done on the gas goes directly into increasing the kinetic energy of its molecules. Higher molecular kinetic energy means higher temperature. This is the same process that occurs in a diesel engine, a bicycle pump, and — on a much grander scale — a collapsing protostellar cloud.

In a protostar, the infalling gas reaches temperatures of ~10⁷ K at the centre, enough to ignite hydrogen fusion. At that point, in Sun-like stars the outward gas (thermal) pressure balances gravity (radiation pressure dominates only in very massive stars) and the star stabilises on the main sequence. Stars less massive than ~0.08 solar masses never reach fusion temperatures — they become brown dwarfs instead.

The ideal-gas law connects the two quantities in this experiment:

For an adiabatic process: T · V^(γ−1) = const, or equivalently T ∝ P^((γ−1)/γ)

Materials

Version 1 — Syringe:

• Large plastic syringe with Luer-lock closure (50–100 mL)
• TI SensorTag (pressure and temperature sensors)
• Smartphone with phyphox

Version 2 — Press-plunger can:

• Glass press-plunger can (French press style)
• Freezer bag tied shut (air-filled ball)
• TI SensorTag
• Smartphone with phyphox

The materials listed are examples. The Luer-lock syringe version is easier to handle in a classroom.

Setup and Procedure

Version 1 — Syringe

1. Pull the plunger fully out of the syringe.
2. Place the TI SensorTag inside the syringe barrel.
3. Push the plunger back in gently.
4. Close the syringe end with the Luer-lock cap.
5. Open pt-star.phyphox in phyphox and connect the SensorTag via Bluetooth.
6. Start the measurement.
7. Compress the syringe quickly and firmly. Keep the Luer-lock cap pointing away from your face.
8. Observe the simultaneous rise in pressure and temperature in the phyphox graph.

Version 2 — Press-plunger can

1. Knot a freezer bag so that it forms a taut, air-filled ball.
2. Place the ball and the SensorTag inside the glass press-plunger can.
3. Open pt-star.phyphox in phyphox and start the measurement.
4. Press down on the plunger. The can is glass — press carefully and not past the point of resistance.

Discussion

• Both pressure and temperature rise together during compression. What does this tell us about the relationship between the two quantities?
• In a stellar nebula, the temperatures required for fusion are ~10 million kelvin. What else must happen beyond simple compression?
• Why do larger, more massive stars form faster than smaller ones?
• A brown dwarf has a mass ~13–80 times that of Jupiter. Why is that threshold physically significant?

phyphox File

• One TI SensorTag (pressure + temperature): pt-star.phyphox

The file contains English, German and French texts. The phyphox files are maintained in the curious-signals-phyphox repository.

Related Publication

Spicker, S. J. & Küpper, A. (2020). Simple DIY experiments on star formation for physics teaching and at home. Institute of Physics Education, University of Cologne.