---
title: "Stellar streams, encounters and observed projections"
description: "Follow orbital clocks from tides to flyby gaps, then recompute a conditional GD-1 comparison and read the fixed 2025 thin-stream and cocoon paper."
canonical_url: "https://sandboxphysics.com/stories/stellar-streams/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/stellar-streams/"
last_modified: "2026-11-05"
---

Stellar streams

# Stellar streams, encounters and observed projections

Sandbox Physics · 2026-11-05

A brief encounter can change orbital clocks long after the force has faded. Start with the physical picture, derive the force and projection, then test both numerical accuracy and real-data model quality.

1. L1

   ## [Why does a stellar stream develop a gap?](https://sandboxphysics.com/stories/stellar-streams/why-stellar-streams-have-gaps/)

   A flyby changes orbital periods. An early pile-up can become a much later sparse region, without uniquely identifying dark matter.

2. L1

   ## [How tides turn a cluster into a stellar stream](https://sandboxphysics.com/stories/stellar-streams/tides-actions-and-many-orbits/)

   Derive a circular-orbit tidal scale, then use phase space, actions and frequency differences to understand why a thin stream contains many orbits.

3. L2

   ## [From a flyby force to a much later stream gap](https://sandboxphysics.com/stories/stellar-streams/from-a-flyby-force-to-a-stream-gap/)

   Derive a Plummer flyby kick, separate the impulse limit from a finite-duration encounter, and follow the resulting changes in orbital clocks.

4. L3

   ## [Project a stellar stream before fitting it](https://sandboxphysics.com/stories/stellar-streams/observing-and-fitting-a-stellar-stream/)

   Define sky coordinates, proper motion and solar reflex, then use known truth and held-out data to separate accurate integration from a good stream model.

5. L3

   ## [Recompute a conditional GD-1 projected-orbit fit](https://sandboxphysics.com/stories/stellar-streams/recompute-gd1-projected-orbit-fit/)

   Run independent stream dynamics, acquire fixed author products, and retain nonconvergence, coverage failures and a poor real-data orbit comparison.

6. L4

   ## [Read the 2025 GD-1 thin-stream and cocoon paper](https://sandboxphysics.com/stories/stellar-streams/reading-gd1-stream-and-cocoon-2025/)

   Follow selection, projections, mixture widths and background alternatives in a fixed DESI paper, separating a broad component from its formation history.

## Sources and calculation scope

1. [Erkal & Belokurov (2015), Forensics of Subhalo-Stream Encounters, arXiv:1412.6035v2](https://arxiv.org/abs/1412.6035v2)

   All 14 pages actually read. Sections 2–3 and equations 1–23 derive the short-encounter circular-stream model; sections 4–6 compare simulations and physical degeneracies. Printed pages 3/9 actually viewed. The imposed warm ensemble is not the paper’s tidal-release calculation.

2. [Valluri et al. (2025), GD-1 Stellar Stream and Cocoon in the DESI Early Data Release, arXiv:2407.06336v2](https://arxiv.org/abs/2407.06336v2)

   Fixed 30 January 2025 v2, 25 pages fully read; original pages 7/12/14/21 actually viewed. Sections 3–5, Figures 2/7–12, Table 3 and Appendix A are distinguished from our conditional orbit exercise. DOI 10.3847/1538-4357/ada690; no original paper, figure or observed row redistributed.

3. [Gala v1.12.0: fixed official GD1Koposov10 coordinate source](https://github.com/adrn/gala/blob/bebac7d728478c5122a568e12175ef894d1c1516/src/gala/coordinates/gd1.py)

   MIT source, fixed commit bebac7d728478c5122a568e12175ef894d1c1516. Literal rounded R and transpose reverse, normalized direction/proper-motion definitions; optional reader fetches and hashes the exact source without installing Gala.

4. [Sandbox Physics standalone stellar-stream scientific reader v1](https://sandboxphysics.com/stories/stellar-streams/materials/stellar-streams-reader-science-v1.zip)

   Own Python integrates 25 finite cases anew, checks an independent high-order reference, half steps, analytic limits and saved-record counts. Default zero network. Optional exact-source galpy exercise retains poor fit/nonconvergence/no-coverage failures. No App/Engine, completed tracks, originals or observed-derived figures in the ZIP.