Lesson 2 of 6 · protostars
Follow a growing protostar
10 minute lesson · 3 checks along the way
Your goal for this lessonExplain how a forming star gains mass and shines before sustained core hydrogen fusion dominates.
Follow a linked name to explore its archive card. “Back to your lesson” brings you to the same part of the reading.
Words with a dotted underline open a short definition right here in the lesson.
Shining before the main sequence
As a collapses, gravitational potential energy is released. Part can become internal energy and part can escape as radiation. A growing central object can therefore shine before sustained hydrogen fusion supplies most of its . Calling it a protostar describes an early evolutionary state, not an object with no energy source. Its surrounding envelope and disk also contribute to the light measured from the unresolved system.
The route is not identical at every mass. A low-mass young object can continue contracting after its main phase, while massive stars can begin core hydrogen fusion while still gaining material. Some low-mass objects never sustain ordinary hydrogen fusion and become . Deuterium fusion can occur in some young objects without making them main-sequence stars. Avoid using any occurrence of a nuclear reaction as the sole boundary between all these categories.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
A disk connects growth with outflow
Infalling material usually carries . It cannot all move straight to the center while retaining that angular momentum unchanged. A disk forms, and stresses, magnetic interactions, and outflows can redistribute angular momentum so that some material moves inward. The disk does not force every parcel to become part of the star. Some remains in the surrounding system; some is expelled, and some may eventually participate in planet formation.
Jets and winds are evidence that growth and loss can happen together. They interact with surrounding gas and may produce bright shock structures far from the central source. The length of a projected jet does not alone provide the 's age: speed, viewing angle, changing activity, and the time over which knots remain visible matter. An outflow traces a process, but it is not a simple ruler for the entire object's history.
Work through a decision
Worked growth: identify the rate you were given
Imagine a protostar initially containing 0.40 . For an idealized 50,000-year interval, its of mass gain is a constant 0.000002 solar masses per year. Multiplying rate by time gives 0.10 solar masses added. The final mass is 0.50 solar masses. Net gain already accounts for the difference between material entering and leaving the defined central object; do not subtract an outflow a second time.
If instead the quoted number were only the disk's inward delivery rate, a separate loss term could be required. Real accretion can vary strongly, so the worked constant rate cannot justify extending the same trend indefinitely. A brief luminous outburst might indicate changed accretion rather than an equivalent increase in stellar mass. Compare repeated observations and models of the star, disk, and envelope before interpreting a brightness change as growth.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Match the observation to the hidden source
The Herschel Space Observatory studied infrared and submillimeter emission, providing ways to investigate cool dust and embedded star formation. Its archive photograph shows the observatory being prepared for environmental testing, not a protostar. The connection is instrumental: detectors must distinguish faint celestial emission from backgrounds, including the observatory itself. A cleanroom image explains where measurements come from; it cannot replace the measurements of a cloud.
For a proposed protostar, combine its spectrum, surrounding material, luminosity, and signs of accretion or outflow. Consider whether a reddened background star or an unresolved companion could imitate part of the signal. Models connect those observations to a likely stage, with uncertainty. The key change toward the is sustained core hydrogen fusion becoming the dominant long-term energy supply, not the moment a previously black point first becomes visible.
The Herschel Space Observatory is raised in the ESTEC cleanroom before transfer into the Large Space Simulator for tests in October 2008. This is observing hardware; its later infrared and submillimeter measurements helped investigate cold material and young stars.
Look closely
What does this photograph document directly, and what evidence would still be needed for a claim about a growing star?
See what to notice
It documents the observatory and its test preparation. A stellar claim needs calibrated celestial measurements and a model of the source, disk, and envelope; no protostar or accretion measurement appears in this photograph.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.